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An equation of the type
 
An equation of the type
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319201.png" /></td> <td valign="top" style="width:5%;text-align:right;">(1)</td></tr></table>
+
$$ \tag{1 }
 +
F ( x, \dots, p _ {i _ {1}  \dots i _ {n} } ,\dots )  = 0 .
 +
$$
 +
 
 +
Here  $  F $
 +
is a given real-valued function of the points  $  x = ( x _ {1}, \dots, x _ {n} ) $
 +
of a domain  $  D $
 +
of a Euclidean space  $  E  ^ {n} $,
 +
$  n\geq  2 $,
 +
and of the real variables
  
Here <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319202.png" /> is a given real-valued function of the points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319203.png" /> of a domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319204.png" /> of a Euclidean space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319205.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319206.png" />, and of the real variables
+
$$
 +
p _ {i _ {1}  \dots i _ {n} }  \equiv 
 +
\frac{\partial  ^ {k} u }{
 +
\partial  x _ {1} ^ {i _ {1} } \dots \partial  x _ {n} ^ {i _ {n} } }
 +
,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319207.png" /></td> </tr></table>
+
where  $  u $
 +
is the unknown function, and where the  $  i _ {1}, \dots, i _ {n} $
 +
are non-negative integer indices,  $  \sum _ {j= 1}  ^ {n} i _ {j} = k $,
 +
$  k = 0, \dots, m $,
 +
$  m \geq  1 $,
 +
and at least one of the derivatives
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319208.png" /> is the unknown function, and where the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d0319209.png" /> are non-negative integer indices, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192010.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192011.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192012.png" />, and at least one of the derivatives
+
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192013.png" /></td> </tr></table>
+
\frac{\partial  F }{\partial  p _ {i _ {1}  \dots i _ {n} } }
 +
,\  \sum _ { j= 1} ^ { n }  i _ {j}  = m ,
 +
$$
  
of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192014.png" /> is non-zero; the natural number <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192015.png" /> is called the order of equation (1).
+
of $  F $
 +
is non-zero; the natural number $  m $
 +
is called the order of equation (1).
  
A regular solution is a function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192016.png" /> defined in the domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192017.png" /> where equation (1) is given, continuous together with its partial derivatives entering the equation and such that (1) holds identically. In the theory of partial differential equations not only regular solutions are important, but also solutions which cease to be regular in a neighbourhood of isolated points or in a neighbourhood of manifolds of special type; in particular, elementary (fundamental) solutions are important. They permit the construction of wide classes of regular solutions (the so-called potentials) and to establish their structural and qualitative properties.
+
A regular solution is a function $  u $
 +
defined in the domain $  D $
 +
where equation (1) is given, continuous together with its partial derivatives entering the equation and such that (1) holds identically. In the theory of partial differential equations not only regular solutions are important, but also solutions which cease to be regular in a neighbourhood of isolated points or in a neighbourhood of manifolds of special type; in particular, elementary (fundamental) solutions are important. They permit the construction of wide classes of regular solutions (the so-called potentials) and to establish their structural and qualitative properties.
  
Under the assumption that the first-order partial derivatives of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192018.png" /> with respect to the variables
+
Under the assumption that the first-order partial derivatives of $  F $
 +
with respect to the variables
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192019.png" /></td> </tr></table>
+
$$
 +
p _ {i _ {1}  \dots i _ {n} } ,\  \sum _ { j= 1} ^ { n }  i _ {j}  = m,
 +
$$
  
are continuous, the following form of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192020.png" />:
+
are continuous, the following form of order $  m $:
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192021.png" /></td> <td valign="top" style="width:5%;text-align:right;">(2)</td></tr></table>
+
$$ \tag{2 }
 +
k ( \lambda _ {1}, \dots, \lambda _ {n} ) =  \sum
 +
\frac{\partial  F }{
 +
\partial  p _ {i _ {1}  \dots i _ {n} } }
 +
\lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } ,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192022.png" /></td> </tr></table>
+
$$
 +
\sum _ { j= 1} ^ { n }  i _ {j}  = m ,
 +
$$
  
with real parameters <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192023.png" />, is known as the characteristic form corresponding to equation (1). It plays a fundamental role in the theory of equations of type (1).
+
with real parameters $  \lambda _ {1}, \dots, \lambda _ {n} $,
 +
is known as the characteristic form corresponding to equation (1). It plays a fundamental role in the theory of equations of type (1).
  
If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192024.png" /> is a linear function in the variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192025.png" />, equation (1) is said to be linear. Linear partial differential equations of the second order may be written as
+
If $  F $
 +
is a linear function in the variables $  p _ {i _ {1}  \dots i _ {n} } $,  
 +
equation (1) is said to be linear. Linear partial differential equations of the second order may be written as
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192026.png" /></td> <td valign="top" style="width:5%;text-align:right;">(3)</td></tr></table>
+
$$ \tag{3 }
 +
\sum _ {i , j = 1 } ^ { n }  A _ {ij}
 +
\frac{\partial  ^ {2} u }{\partial  x _ {i} \partial  x _ {j} }
 +
+ \sum _ { j= 1} ^ { n }  B _ {j}
 +
\frac{\partial  u }{
 +
\partial  x _ {j} }
 +
+ C u  = f ,
 +
$$
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192027.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192028.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192029.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192030.png" /> are real-valued functions on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192031.png" />. Equation (3) is said to be homogeneous if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192032.png" /> for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192033.png" />. In the case of equation (3), the form (2) is quadratic:
+
where $  A _ {ij} $,  
 +
$  B _ {j} $,  
 +
$  C $,  
 +
and $  f $
 +
are real-valued functions on $  D $.  
 +
Equation (3) is said to be homogeneous if $  f ( x) = 0 $
 +
for all $  x \in D $.  
 +
In the case of equation (3), the form (2) is quadratic:
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192034.png" /></td> </tr></table>
+
$$
 +
Q ( \lambda _ {1}, \dots, \lambda _ {n} )  = \sum _ {i , j = 1 } ^ { n }  A _ {ij} \lambda _ {i} \lambda _ {j} ,
 +
$$
  
with coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192035.png" /> which only depend on the point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192036.png" />. At each such point the quadratic form <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192037.png" /> may be reduced, by a non-singular affine transformation of the variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192038.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192039.png" />, to the canonical form
+
with coefficients $  A _ {ij} $
 +
which only depend on the point $  x \in D $.  
 +
At each such point the quadratic form $  Q $
 +
may be reduced, by a non-singular affine transformation of the variables $  \lambda _ {i} = \lambda _ {i} ( \xi _ {1}, \dots, \xi _ {n} ) $,
 +
$  i = 1, \dots, n $,  
 +
to the canonical form
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192040.png" /></td> </tr></table>
+
$$
 +
= \sum _ { i= 1} ^ { n }  \alpha _ {i} \xi _ {i}  ^ {2} ,
 +
$$
  
where the coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192041.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192042.png" />, assume the values <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192043.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192044.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192045.png" />, and the number of negative coefficients (the index of inertia) and the number of zero coefficients (the defect of the form) are affine invariants. If all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192046.png" /> or if all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192047.png" />, i.e. if the form <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192048.png" /> is positive or negative definite, respectively, equation (3) is called elliptic at the point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192049.png" />. If one of the coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192050.png" /> is negative, while all the others are positive (or vice versa), equation (3) is called hyperbolic at <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192051.png" />. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192052.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192053.png" />, of the coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192054.png" /> are positive, whereas the remaining <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192055.png" /> are negative, equation (3) is called ultra-hyperbolic. If at least one (but not all) of these coefficients vanishes, equation (3) is called parabolic at <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192056.png" />. One says that, in its domain of definition <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192057.png" />, equation (3) is of elliptic, hyperbolic or parabolic type if it is elliptic, hyperbolic or parabolic, respectively, at every point of this domain. An elliptic equation (3) in a domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192058.png" /> is called uniformly elliptic if there exist real numbers <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192059.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192060.png" /> of the same sign such that
+
where the coefficients $  \alpha _ {i} $,
 +
$  i = 1, \dots, n $,  
 +
assume the values $  1 $,  
 +
$  - 1 $,  
 +
0 $,  
 +
and the number of negative coefficients (the index of inertia) and the number of zero coefficients (the defect of the form) are affine invariants. If all $  \alpha _ {i} = 1 $
 +
or if all $  \alpha _ {i} = - 1 $,  
 +
i.e. if the form $  Q $
 +
is positive or negative definite, respectively, equation (3) is called elliptic at the point $  x \in D $.  
 +
If one of the coefficients $  \alpha _ {i} $
 +
is negative, while all the others are positive (or vice versa), equation (3) is called hyperbolic at $  x $.  
 +
If $  l $,
 +
$  1 < l < n- 1 $,  
 +
of the coefficients $  \alpha _ {i} $
 +
are positive, whereas the remaining $  n- l $
 +
are negative, equation (3) is called ultra-hyperbolic. If at least one (but not all) of these coefficients vanishes, equation (3) is called parabolic at $  x $.  
 +
One says that, in its domain of definition $  D $,  
 +
equation (3) is of elliptic, hyperbolic or parabolic type if it is elliptic, hyperbolic or parabolic, respectively, at every point of this domain. An elliptic equation (3) in a domain $  D $
 +
is called uniformly elliptic if there exist real numbers $  k _ {0} $
 +
and $  k _ {1} $
 +
of the same sign such that
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192061.png" /></td> </tr></table>
+
$$
 +
k _ {0} \sum _ { i= 1} ^ { n }  \lambda _ {i}  ^ {2}  \leq  Q ( \lambda _ {1}, \dots, \lambda _ {n} )  \leq  k _ {1} \sum _ { i= 1} ^ { n }  \lambda _ {i}  ^ {2}
 +
$$
  
for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192062.png" />. If equation (3) is of different types in different parts of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192063.png" />, one says that it is an equation of mixed type in this region.
+
for all $  x \in D $.  
 +
If equation (3) is of different types in different parts of $  D $,  
 +
one says that it is an equation of mixed type in this region.
  
 
The [[Laplace equation|Laplace equation]]
 
The [[Laplace equation|Laplace equation]]
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192064.png" /></td> </tr></table>
+
$$
 +
\sum _ { i= 1} ^ { n } 
 +
\frac{\partial  ^ {2} u }{\partial  x _ {i}  ^ {2} }
 +
  = 0 ,
 +
$$
  
 
the [[Thermal-conductance equation|thermal-conductance equation]]
 
the [[Thermal-conductance equation|thermal-conductance equation]]
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192065.png" /></td> </tr></table>
+
$$
 +
\sum _ { i= 1} ^ { n-  1}
 +
\frac{\partial  ^ {2} u }{\partial  x _ {i}  ^ {2} }
 +
-  
 +
\frac{\partial  u }{\partial  x _ {n} }
 +
  = 0 ,
 +
$$
  
 
and the [[Wave equation|wave equation]]
 
and the [[Wave equation|wave equation]]
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192066.png" /></td> </tr></table>
+
$$
 +
\sum _ { i= 1} ^ { n-  1}
 +
\frac{\partial  ^ {2} u }{\partial  x _ {i}  ^ {2} }
 +
-  
 +
\frac{
 +
\partial  ^ {2} u }{\partial  x _ {n}  ^ {2} }
 +
  = 0 ,
 +
$$
  
 
are typical examples of linear second-order elliptic, parabolic and hyperbolic equations, respectively. For more details see [[Linear hyperbolic partial differential equation and system|Linear hyperbolic partial differential equation and system]]; [[Linear parabolic partial differential equation and system|Linear parabolic partial differential equation and system]]; [[Linear elliptic partial differential equation and system|Linear elliptic partial differential equation and system]].
 
are typical examples of linear second-order elliptic, parabolic and hyperbolic equations, respectively. For more details see [[Linear hyperbolic partial differential equation and system|Linear hyperbolic partial differential equation and system]]; [[Linear parabolic partial differential equation and system|Linear parabolic partial differential equation and system]]; [[Linear elliptic partial differential equation and system|Linear elliptic partial differential equation and system]].
Line 61: Line 175:
 
The [[Tricomi equation|Tricomi equation]]
 
The [[Tricomi equation|Tricomi equation]]
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192067.png" /></td> </tr></table>
+
$$
 +
x _ {2}
 +
\frac{\partial  ^ {2} u }{\partial  x _ {1}  ^ {2} }
 +
+
 +
\frac{\partial  ^ {2} u
 +
}{\partial  x _ {2}  ^ {2} }
 +
  = 0
 +
$$
  
is an equation of mixed type in any domain of the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192068.png" />-plane whose intersection with the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192069.png" /> axis is non-empty (for more details see [[Mixed-type differential equation|Mixed-type differential equation]]).
+
is an equation of mixed type in any domain of the $  ( x _ {1} , x _ {2} ) $-plane whose intersection with the $  x _ {2} = 0 $
 +
axis is non-empty (for more details see [[Mixed-type differential equation|Mixed-type differential equation]]).
  
In the case of a linear partial differential equation of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192070.png" />,
+
In the case of a linear partial differential equation of order $  m $,
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192071.png" /></td> <td valign="top" style="width:5%;text-align:right;">(4)</td></tr></table>
+
$$ \tag{4 }
 +
\sum \alpha _ {i _ {1}  \dots i _ {n} } ( x )  
 +
\frac{\partial  ^ {m} u }{\partial  x _ {1} ^ {i _ {1} } \dots \partial  x _ {n} ^ {i _ {n} } }
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192072.png" /> is a linear partial differential operator of order lower than <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192073.png" />, the form (2) looks like:
+
+ L _ {1} u  = f ,\  \sum _ { j= 1} ^ { n }  i _ {j} = m ,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192074.png" /></td> <td valign="top" style="width:5%;text-align:right;">(5)</td></tr></table>
+
where  $  L _ {1} $
 +
is a linear partial differential operator of order lower than  $  m $,
 +
the form (2) looks like:
  
If, for a given value of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192075.png" />, it is possible to find an affine transformation <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192076.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192077.png" />, as a result of which the form obtained from (5) contains only <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192078.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192079.png" />, variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192080.png" />, then one says that equation (4) becomes parabolically degenerate at <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192081.png" />. If parabolic degeneration is absent and if the conical manifold
+
$$ \tag{5 }
 +
k ( \lambda _ {1}, \dots, \lambda _ {n} ) = \sum \alpha _ {i _ {1} \dots i _ {n}  } ( x ) \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } .
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192082.png" /></td> <td valign="top" style="width:5%;text-align:right;">(6)</td></tr></table>
+
If, for a given value of  $  x \in D $,
 +
it is possible to find an affine transformation  $  \lambda _ {i} = \lambda _ {i} ( \mu _ {i}, \dots, \mu _ {n} ) $,
 +
$  i = 1, \dots, n $,
 +
as a result of which the form obtained from (5) contains only  $  l $,
 +
0 < l < n $,
 +
variables  $  \mu $,
 +
then one says that equation (4) becomes parabolically degenerate at  $  x $.
 +
If parabolic degeneration is absent and if the conical manifold
  
has no real points other than <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192083.png" />, equation (4) is called elliptic at the point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192084.png" />. Equation (4) is called hyperbolic at <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192085.png" /> if in the space of variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192086.png" /> there exists a straight line <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192087.png" /> such that if it is accepted as a coordinate line in the new variables <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192088.png" /> obtained by an affine transformation of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192089.png" />, equation (6) will have, with respect to the coordinate varying along <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192090.png" />, exactly <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192091.png" /> real roots (simple or multiple) for any choice of the remaining coordinates <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192092.png" />.
+
$$ \tag{6 }
 +
k ( \lambda _ {1}, \dots, \lambda _ {n} ) = 0
 +
$$
  
The classification by type of equation (1) takes place in a similar manner in the non-linear case, by the character of the form (2). Since the coefficients of the form (2) depend, besides on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192093.png" />, now also on the solution sought and on its derivatives, the classification by type makes sense for this solution only. See also [[Non-linear partial differential equation|Non-linear partial differential equation]].
+
has no real points other than  $  \lambda _ {1} = \dots = \lambda _ {n} = 0 $,
 +
equation (4) is called elliptic at the point  $  x $.
 +
Equation (4) is called hyperbolic at  $  x $
 +
if in the space of variables  $  \lambda _ {1}, \dots, \lambda _ {n} $
 +
there exists a straight line  $  \delta $
 +
such that if it is accepted as a coordinate line in the new variables  $  \mu _ {1}, \dots, \mu _ {n} $
 +
obtained by an affine transformation of $  \lambda _ {1}, \dots, \lambda _ {n} $,
 +
equation (6) will have, with respect to the coordinate varying along  $  \delta $,
 +
exactly  $  m $
 +
real roots (simple or multiple) for any choice of the remaining coordinates  $  \mu $.
  
If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192094.png" /> is an <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192095.png" />-dimensional vector <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192096.png" /> with components
+
The classification by type of equation (1) takes place in a similar manner in the non-linear case, by the character of the form (2). Since the coefficients of the form (2) depend, besides on  $  x $,
 +
now also on the solution sought and on its derivatives, the classification by type makes sense for this solution only. See also [[Non-linear partial differential equation|Non-linear partial differential equation]].
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192097.png" /></td> </tr></table>
+
If  $  F $
 +
is an  $  N $-dimensional vector  $  F = ( F _ {1}, \dots, F _ {N} ) $
 +
with components
  
depending on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192098.png" /> and on the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d03192099.png" />-dimensional vectors
+
$$
 +
F _ {i} ( x, \dots, p _ {i _ {1}  \dots i _ {n} } ,\dots ) ,\  i =
 +
1, \dots, N ,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920100.png" /></td> </tr></table>
+
depending on  $  x \in D $
 +
and on the  $  M $-dimensional vectors
  
the vector equation (1) is said to be a system of partial differential equations for the unknown functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920101.png" /> or for the unknown vector <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920102.png" />. The highest order of the derivatives of the unknown functions entering the equation of the system is called the order of this system (equation). If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920103.png" /> and the order of each equation of the system (1) is <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920104.png" />, the determinant
+
$$
 +
p _ {i _ {1}  \dots i _ {n} }  = ( p _ {i _ {1}  \dots i _ {n} }  ^ {1} \dots p _ {i _ {1}  \dots i _ {n} }  ^ {M} ) =
 +
\frac{
 +
\partial  ^ {k} u }{\partial  x _ {1} ^ {i _ {1} } \dots \partial  x _ {n} ^ {i _ {n} } }
 +
,
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920105.png" /></td> <td valign="top" style="width:5%;text-align:right;">(7)</td></tr></table>
+
the vector equation (1) is said to be a system of partial differential equations for the unknown functions  $  u _ {1}, \dots, u _ {M} $
 +
or for the unknown vector  $  u = ( u _ {1}, \dots, u _ {M} ) $.  
 +
The highest order of the derivatives of the unknown functions entering the equation of the system is called the order of this system (equation). If  $  M = N $
 +
and the order of each equation of the system (1) is  $  m $,
 +
the determinant
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920106.png" /></td> </tr></table>
+
$$ \tag{7 }
 +
k ( \lambda _ {1}, \dots, \lambda _ {n} ) =
 +
$$
 +
 
 +
$$
 +
= \
 +
\mathop{\rm det}  \sum _ {i _ {1}, \dots, i _ {n} }
 +
\left \|
 +
\frac{\partial  F _ {i} }{\partial  p _ {i _ {1}  \dots
 +
i _ {n} }  ^ {j} }
 +
\right \| \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } ,
 +
$$
  
 
where
 
where
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920107.png" /></td> </tr></table>
+
$$
 +
\left \|
 +
\frac{\partial  F _ {i} }{\partial  p _ {i _ {1}  \dots i _ {n}  }  ^ {j} }
 +
\right \| ,\  i , j = 1, \dots, N ,\  \sum _ { k= 1} ^ { n }
 +
i _ {k} = m ,
 +
$$
  
is a square matrix, is a form of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920108.png" /> with respect to the real scalar parameters <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920109.png" />, known as the characteristic determinant of the system (1). The classification by type of the system (1) is effected by the character of (7) exactly as for a single equation of order <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920110.png" />. The quantities appearing on the left-hand side of equation (1) may be complex numbers and functions. A complex partial differential equation is replaced by a system of real equations in an obvious manner.
+
is a square matrix, is a form of order $  Nm $
 +
with respect to the real scalar parameters $  \lambda _ {1}, \dots, \lambda _ {n} $,  
 +
known as the characteristic determinant of the system (1). The classification by type of the system (1) is effected by the character of (7) exactly as for a single equation of order $  m $.  
 +
The quantities appearing on the left-hand side of equation (1) may be complex numbers and functions. A complex partial differential equation is replaced by a system of real equations in an obvious manner.
  
 
A partial differential equation need not have any solution at all. However, equations which are used in practical applications usually have entire families of solutions. When such equations are derived from the general laws governing natural phenomena, additional conditions on the solutions sought naturally arise. Finding regular solutions satisfying these conditions is the principal task of the theory of partial differential equations. The nature of such conditions depends largely on the type of the equation under consideration.
 
A partial differential equation need not have any solution at all. However, equations which are used in practical applications usually have entire families of solutions. When such equations are derived from the general laws governing natural phenomena, additional conditions on the solutions sought naturally arise. Finding regular solutions satisfying these conditions is the principal task of the theory of partial differential equations. The nature of such conditions depends largely on the type of the equation under consideration.
  
For elliptic equations one usually studies the so-called boundary value problem which may in principle be formulated as follows (cf. [[Boundary value problem, elliptic equations|Boundary value problem, elliptic equations]]): To find, in a domain <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920111.png" />, a regular solution <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920112.png" /> of equation (1) satisfying the condition
+
For elliptic equations one usually studies the so-called boundary value problem which may in principle be formulated as follows (cf. [[Boundary value problem, elliptic equations|Boundary value problem, elliptic equations]]): To find, in a domain $  D $,  
 +
a regular solution $  u $
 +
of equation (1) satisfying the condition
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920113.png" /></td> <td valign="top" style="width:5%;text-align:right;">(8)</td></tr></table>
+
$$ \tag{8 }
 +
f \left ( x , u, \dots,
 +
\frac{\partial  ^ {l} u }{\partial  x _ {1} ^ {i _ {1} } \dots \partial  x _ {n} ^ {i _ {n} } }
 +
,\dots \right ) +
 +
$$
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920114.png" /></td> </tr></table>
+
$$
 +
+
 +
\int\limits _ { S } H \left ( x , t , u ( t), \dots,
 +
\frac{\partial  ^ {l} u ( t) }{\partial  t _ {1} ^ {i _ {1} } \dots \partial  t _ {n} ^ {i _ {n} } }
 +
,\dots \right )  d s _ {t}  = 0 ,
 +
$$
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920115.png" /> is the boundary of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920116.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920117.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920118.png" /> are given real-valued functions, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920119.png" /> is the area element of the surface <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920120.png" />, while
+
where $  S $
 +
is the boundary of $  D $,  
 +
$  f $
 +
and $  H $
 +
are given real-valued functions, d s _ {t} $
 +
is the area element of the surface $  S $,  
 +
while
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920121.png" /></td> </tr></table>
+
$$
  
are understood to be the respective derivatives of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920122.png" /> obtained as limits from the inside of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920123.png" /> towards <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920124.png" />.
+
\frac{\partial  ^ {l} u }{\partial  x _ {1} ^ {i _ {1} } \dots \partial  x _ {n} ^ {i _ {n} } }
 +
,\  \sum _ { j= 1} ^ { n }  i _ {j} = l ,\ \
 +
l < m ,
 +
$$
 +
 
 +
are understood to be the respective derivatives of $  u $
 +
obtained as limits from the inside of $  D $
 +
towards $  S $.
  
 
If posed in this general manner, problem (8) is still far from being completely solved. Special cases of this problem — viz. the so-called first- and second-order boundary value problems (cf. [[Dirichlet problem|Dirichlet problem]] and [[Neumann problem]]) for the case of second-order linear uniformly-elliptic equations — have been studied in greater detail.
 
If posed in this general manner, problem (8) is still far from being completely solved. Special cases of this problem — viz. the so-called first- and second-order boundary value problems (cf. [[Dirichlet problem|Dirichlet problem]] and [[Neumann problem]]) for the case of second-order linear uniformly-elliptic equations — have been studied in greater detail.
  
In the boundary value problems for elliptic equations, any boundary of the region of the solution may serve as the support of the data. By contrast, in the case of broad classes of equations of hyperbolic and parabolic type non-closed oriented surfaces of the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/d/d031/d031920/d031920125.png" /> carry the supplementary data, and the domain of definition of the solution substantially depends on these surfaces. These include, for example, the [[Cauchy problem|Cauchy problem]] with initial data and the characteristic Cauchy problem (cf. [[Cauchy characteristic problem|Cauchy characteristic problem]]). Boundary value problems for equations of mixed type are posed in a special manner. In the theory of partial differential equations the extensive class of mixed problems has aroused much interest. See [[Mixed and boundary value problems for hyperbolic equations and systems|Mixed and boundary value problems for hyperbolic equations and systems]]; [[Mixed and boundary value problems for parabolic equations and systems|Mixed and boundary value problems for parabolic equations and systems]].
+
In the boundary value problems for elliptic equations, any boundary of the region of the solution may serve as the support of the data. By contrast, in the case of broad classes of equations of hyperbolic and parabolic type non-closed oriented surfaces of the space $  E  ^ {n} $
 +
carry the supplementary data, and the domain of definition of the solution substantially depends on these surfaces. These include, for example, the [[Cauchy problem|Cauchy problem]] with initial data and the characteristic Cauchy problem (cf. [[Cauchy characteristic problem|Cauchy characteristic problem]]). Boundary value problems for equations of mixed type are posed in a special manner. In the theory of partial differential equations the extensive class of mixed problems has aroused much interest. See [[Mixed and boundary value problems for hyperbolic equations and systems|Mixed and boundary value problems for hyperbolic equations and systems]]; [[Mixed and boundary value problems for parabolic equations and systems|Mixed and boundary value problems for parabolic equations and systems]].
  
 
A problem is considered to be well-posed in the classical sense if it has a unique solution which depends continuously on the data of the problem. Until recently, problems which did not satisfy this requirement were considered meaningless. Since the 1940s, the broad range of mathematical problems in physics, mechanics and technology made it imperative not only to extend the concept of well-posedness of problems involving partial differential equations, but also to extend the meaning of the concept of a solution. So-called generalized solutions were introduced. Beside the question of the existence and uniqueness of exact solutions of problems involving partial differential equations, the concept of approximation of solutions and methods for practical computation have become important in applications.
 
A problem is considered to be well-posed in the classical sense if it has a unique solution which depends continuously on the data of the problem. Until recently, problems which did not satisfy this requirement were considered meaningless. Since the 1940s, the broad range of mathematical problems in physics, mechanics and technology made it imperative not only to extend the concept of well-posedness of problems involving partial differential equations, but also to extend the meaning of the concept of a solution. So-called generalized solutions were introduced. Beside the question of the existence and uniqueness of exact solutions of problems involving partial differential equations, the concept of approximation of solutions and methods for practical computation have become important in applications.
Line 133: Line 340:
 
====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  L. Bers,  F. John,  M. Schechter,  "Partial differential equations" , Interscience  (1964)  {{MR|0163043}} {{ZBL|0126.00207}} </TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  A.V. Bitsadze,  "The equations of mathematical physics" , MIR  (1980)  (Translated from Russian)  {{MR|0581247}} {{ZBL|0499.35002}} </TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  V.S. Vladimirov,  "Equations of mathematical physics" , MIR  (1984)  (Translated from Russian)  {{MR|0764399}} {{ZBL|0954.35001}} {{ZBL|0652.35002}} {{ZBL|0695.35001}} {{ZBL|0699.35005}} {{ZBL|0607.35001}} {{ZBL|0506.35001}} {{ZBL|0223.35002}} {{ZBL|0231.35002}} {{ZBL|0207.09101}} </TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  R. Courant,  D. Hilbert,  "Methods of mathematical physics. Partial differential equations" , '''2''' , Interscience  (1965)  (Translated from German)  {{MR|0195654}} {{ZBL|}} </TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  A.N. Tikhonov,  A.A. Samarskii,  "Equations of mathematical physics" , Pergamon  (1963)  (Translated from Russian)  {{MR|0165209}} {{ZBL|0111.29008}} </TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  L. Bers,  F. John,  M. Schechter,  "Partial differential equations" , Interscience  (1964)  {{MR|0163043}} {{ZBL|0126.00207}} </TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  A.V. Bitsadze,  "The equations of mathematical physics" , MIR  (1980)  (Translated from Russian)  {{MR|0581247}} {{ZBL|0499.35002}} </TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  V.S. Vladimirov,  "Equations of mathematical physics" , MIR  (1984)  (Translated from Russian)  {{MR|0764399}} {{ZBL|0954.35001}} {{ZBL|0652.35002}} {{ZBL|0695.35001}} {{ZBL|0699.35005}} {{ZBL|0607.35001}} {{ZBL|0506.35001}} {{ZBL|0223.35002}} {{ZBL|0231.35002}} {{ZBL|0207.09101}} </TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  R. Courant,  D. Hilbert,  "Methods of mathematical physics. Partial differential equations" , '''2''' , Interscience  (1965)  (Translated from German)  {{MR|0195654}} {{ZBL|}} </TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  A.N. Tikhonov,  A.A. Samarskii,  "Equations of mathematical physics" , Pergamon  (1963)  (Translated from Russian)  {{MR|0165209}} {{ZBL|0111.29008}} </TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====

Latest revision as of 02:26, 23 January 2022


An equation of the type

$$ \tag{1 } F ( x, \dots, p _ {i _ {1} \dots i _ {n} } ,\dots ) = 0 . $$

Here $ F $ is a given real-valued function of the points $ x = ( x _ {1}, \dots, x _ {n} ) $ of a domain $ D $ of a Euclidean space $ E ^ {n} $, $ n\geq 2 $, and of the real variables

$$ p _ {i _ {1} \dots i _ {n} } \equiv \frac{\partial ^ {k} u }{ \partial x _ {1} ^ {i _ {1} } \dots \partial x _ {n} ^ {i _ {n} } } , $$

where $ u $ is the unknown function, and where the $ i _ {1}, \dots, i _ {n} $ are non-negative integer indices, $ \sum _ {j= 1} ^ {n} i _ {j} = k $, $ k = 0, \dots, m $, $ m \geq 1 $, and at least one of the derivatives

$$ \frac{\partial F }{\partial p _ {i _ {1} \dots i _ {n} } } ,\ \sum _ { j= 1} ^ { n } i _ {j} = m , $$

of $ F $ is non-zero; the natural number $ m $ is called the order of equation (1).

A regular solution is a function $ u $ defined in the domain $ D $ where equation (1) is given, continuous together with its partial derivatives entering the equation and such that (1) holds identically. In the theory of partial differential equations not only regular solutions are important, but also solutions which cease to be regular in a neighbourhood of isolated points or in a neighbourhood of manifolds of special type; in particular, elementary (fundamental) solutions are important. They permit the construction of wide classes of regular solutions (the so-called potentials) and to establish their structural and qualitative properties.

Under the assumption that the first-order partial derivatives of $ F $ with respect to the variables

$$ p _ {i _ {1} \dots i _ {n} } ,\ \sum _ { j= 1} ^ { n } i _ {j} = m, $$

are continuous, the following form of order $ m $:

$$ \tag{2 } k ( \lambda _ {1}, \dots, \lambda _ {n} ) = \sum \frac{\partial F }{ \partial p _ {i _ {1} \dots i _ {n} } } \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } , $$

$$ \sum _ { j= 1} ^ { n } i _ {j} = m , $$

with real parameters $ \lambda _ {1}, \dots, \lambda _ {n} $, is known as the characteristic form corresponding to equation (1). It plays a fundamental role in the theory of equations of type (1).

If $ F $ is a linear function in the variables $ p _ {i _ {1} \dots i _ {n} } $, equation (1) is said to be linear. Linear partial differential equations of the second order may be written as

$$ \tag{3 } \sum _ {i , j = 1 } ^ { n } A _ {ij} \frac{\partial ^ {2} u }{\partial x _ {i} \partial x _ {j} } + \sum _ { j= 1} ^ { n } B _ {j} \frac{\partial u }{ \partial x _ {j} } + C u = f , $$

where $ A _ {ij} $, $ B _ {j} $, $ C $, and $ f $ are real-valued functions on $ D $. Equation (3) is said to be homogeneous if $ f ( x) = 0 $ for all $ x \in D $. In the case of equation (3), the form (2) is quadratic:

$$ Q ( \lambda _ {1}, \dots, \lambda _ {n} ) = \sum _ {i , j = 1 } ^ { n } A _ {ij} \lambda _ {i} \lambda _ {j} , $$

with coefficients $ A _ {ij} $ which only depend on the point $ x \in D $. At each such point the quadratic form $ Q $ may be reduced, by a non-singular affine transformation of the variables $ \lambda _ {i} = \lambda _ {i} ( \xi _ {1}, \dots, \xi _ {n} ) $, $ i = 1, \dots, n $, to the canonical form

$$ Q = \sum _ { i= 1} ^ { n } \alpha _ {i} \xi _ {i} ^ {2} , $$

where the coefficients $ \alpha _ {i} $, $ i = 1, \dots, n $, assume the values $ 1 $, $ - 1 $, $ 0 $, and the number of negative coefficients (the index of inertia) and the number of zero coefficients (the defect of the form) are affine invariants. If all $ \alpha _ {i} = 1 $ or if all $ \alpha _ {i} = - 1 $, i.e. if the form $ Q $ is positive or negative definite, respectively, equation (3) is called elliptic at the point $ x \in D $. If one of the coefficients $ \alpha _ {i} $ is negative, while all the others are positive (or vice versa), equation (3) is called hyperbolic at $ x $. If $ l $, $ 1 < l < n- 1 $, of the coefficients $ \alpha _ {i} $ are positive, whereas the remaining $ n- l $ are negative, equation (3) is called ultra-hyperbolic. If at least one (but not all) of these coefficients vanishes, equation (3) is called parabolic at $ x $. One says that, in its domain of definition $ D $, equation (3) is of elliptic, hyperbolic or parabolic type if it is elliptic, hyperbolic or parabolic, respectively, at every point of this domain. An elliptic equation (3) in a domain $ D $ is called uniformly elliptic if there exist real numbers $ k _ {0} $ and $ k _ {1} $ of the same sign such that

$$ k _ {0} \sum _ { i= 1} ^ { n } \lambda _ {i} ^ {2} \leq Q ( \lambda _ {1}, \dots, \lambda _ {n} ) \leq k _ {1} \sum _ { i= 1} ^ { n } \lambda _ {i} ^ {2} $$

for all $ x \in D $. If equation (3) is of different types in different parts of $ D $, one says that it is an equation of mixed type in this region.

The Laplace equation

$$ \sum _ { i= 1} ^ { n } \frac{\partial ^ {2} u }{\partial x _ {i} ^ {2} } = 0 , $$

the thermal-conductance equation

$$ \sum _ { i= 1} ^ { n- 1} \frac{\partial ^ {2} u }{\partial x _ {i} ^ {2} } - \frac{\partial u }{\partial x _ {n} } = 0 , $$

and the wave equation

$$ \sum _ { i= 1} ^ { n- 1} \frac{\partial ^ {2} u }{\partial x _ {i} ^ {2} } - \frac{ \partial ^ {2} u }{\partial x _ {n} ^ {2} } = 0 , $$

are typical examples of linear second-order elliptic, parabolic and hyperbolic equations, respectively. For more details see Linear hyperbolic partial differential equation and system; Linear parabolic partial differential equation and system; Linear elliptic partial differential equation and system.

The Tricomi equation

$$ x _ {2} \frac{\partial ^ {2} u }{\partial x _ {1} ^ {2} } + \frac{\partial ^ {2} u }{\partial x _ {2} ^ {2} } = 0 $$

is an equation of mixed type in any domain of the $ ( x _ {1} , x _ {2} ) $-plane whose intersection with the $ x _ {2} = 0 $ axis is non-empty (for more details see Mixed-type differential equation).

In the case of a linear partial differential equation of order $ m $,

$$ \tag{4 } \sum \alpha _ {i _ {1} \dots i _ {n} } ( x ) \frac{\partial ^ {m} u }{\partial x _ {1} ^ {i _ {1} } \dots \partial x _ {n} ^ {i _ {n} } } + L _ {1} u = f ,\ \sum _ { j= 1} ^ { n } i _ {j} = m , $$

where $ L _ {1} $ is a linear partial differential operator of order lower than $ m $, the form (2) looks like:

$$ \tag{5 } k ( \lambda _ {1}, \dots, \lambda _ {n} ) = \sum \alpha _ {i _ {1} \dots i _ {n} } ( x ) \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } . $$

If, for a given value of $ x \in D $, it is possible to find an affine transformation $ \lambda _ {i} = \lambda _ {i} ( \mu _ {i}, \dots, \mu _ {n} ) $, $ i = 1, \dots, n $, as a result of which the form obtained from (5) contains only $ l $, $ 0 < l < n $, variables $ \mu $, then one says that equation (4) becomes parabolically degenerate at $ x $. If parabolic degeneration is absent and if the conical manifold

$$ \tag{6 } k ( \lambda _ {1}, \dots, \lambda _ {n} ) = 0 $$

has no real points other than $ \lambda _ {1} = \dots = \lambda _ {n} = 0 $, equation (4) is called elliptic at the point $ x $. Equation (4) is called hyperbolic at $ x $ if in the space of variables $ \lambda _ {1}, \dots, \lambda _ {n} $ there exists a straight line $ \delta $ such that if it is accepted as a coordinate line in the new variables $ \mu _ {1}, \dots, \mu _ {n} $ obtained by an affine transformation of $ \lambda _ {1}, \dots, \lambda _ {n} $, equation (6) will have, with respect to the coordinate varying along $ \delta $, exactly $ m $ real roots (simple or multiple) for any choice of the remaining coordinates $ \mu $.

The classification by type of equation (1) takes place in a similar manner in the non-linear case, by the character of the form (2). Since the coefficients of the form (2) depend, besides on $ x $, now also on the solution sought and on its derivatives, the classification by type makes sense for this solution only. See also Non-linear partial differential equation.

If $ F $ is an $ N $-dimensional vector $ F = ( F _ {1}, \dots, F _ {N} ) $ with components

$$ F _ {i} ( x, \dots, p _ {i _ {1} \dots i _ {n} } ,\dots ) ,\ i = 1, \dots, N , $$

depending on $ x \in D $ and on the $ M $-dimensional vectors

$$ p _ {i _ {1} \dots i _ {n} } = ( p _ {i _ {1} \dots i _ {n} } ^ {1} \dots p _ {i _ {1} \dots i _ {n} } ^ {M} ) = \frac{ \partial ^ {k} u }{\partial x _ {1} ^ {i _ {1} } \dots \partial x _ {n} ^ {i _ {n} } } , $$

the vector equation (1) is said to be a system of partial differential equations for the unknown functions $ u _ {1}, \dots, u _ {M} $ or for the unknown vector $ u = ( u _ {1}, \dots, u _ {M} ) $. The highest order of the derivatives of the unknown functions entering the equation of the system is called the order of this system (equation). If $ M = N $ and the order of each equation of the system (1) is $ m $, the determinant

$$ \tag{7 } k ( \lambda _ {1}, \dots, \lambda _ {n} ) = $$

$$ = \ \mathop{\rm det} \sum _ {i _ {1}, \dots, i _ {n} } \left \| \frac{\partial F _ {i} }{\partial p _ {i _ {1} \dots i _ {n} } ^ {j} } \right \| \lambda _ {1} ^ {i _ {1} } \dots \lambda _ {n} ^ {i _ {n} } , $$

where

$$ \left \| \frac{\partial F _ {i} }{\partial p _ {i _ {1} \dots i _ {n} } ^ {j} } \right \| ,\ i , j = 1, \dots, N ,\ \sum _ { k= 1} ^ { n } i _ {k} = m , $$

is a square matrix, is a form of order $ Nm $ with respect to the real scalar parameters $ \lambda _ {1}, \dots, \lambda _ {n} $, known as the characteristic determinant of the system (1). The classification by type of the system (1) is effected by the character of (7) exactly as for a single equation of order $ m $. The quantities appearing on the left-hand side of equation (1) may be complex numbers and functions. A complex partial differential equation is replaced by a system of real equations in an obvious manner.

A partial differential equation need not have any solution at all. However, equations which are used in practical applications usually have entire families of solutions. When such equations are derived from the general laws governing natural phenomena, additional conditions on the solutions sought naturally arise. Finding regular solutions satisfying these conditions is the principal task of the theory of partial differential equations. The nature of such conditions depends largely on the type of the equation under consideration.

For elliptic equations one usually studies the so-called boundary value problem which may in principle be formulated as follows (cf. Boundary value problem, elliptic equations): To find, in a domain $ D $, a regular solution $ u $ of equation (1) satisfying the condition

$$ \tag{8 } f \left ( x , u, \dots, \frac{\partial ^ {l} u }{\partial x _ {1} ^ {i _ {1} } \dots \partial x _ {n} ^ {i _ {n} } } ,\dots \right ) + $$

$$ + \int\limits _ { S } H \left ( x , t , u ( t), \dots, \frac{\partial ^ {l} u ( t) }{\partial t _ {1} ^ {i _ {1} } \dots \partial t _ {n} ^ {i _ {n} } } ,\dots \right ) d s _ {t} = 0 , $$

where $ S $ is the boundary of $ D $, $ f $ and $ H $ are given real-valued functions, $ d s _ {t} $ is the area element of the surface $ S $, while

$$ \frac{\partial ^ {l} u }{\partial x _ {1} ^ {i _ {1} } \dots \partial x _ {n} ^ {i _ {n} } } ,\ \sum _ { j= 1} ^ { n } i _ {j} = l ,\ \ l < m , $$

are understood to be the respective derivatives of $ u $ obtained as limits from the inside of $ D $ towards $ S $.

If posed in this general manner, problem (8) is still far from being completely solved. Special cases of this problem — viz. the so-called first- and second-order boundary value problems (cf. Dirichlet problem and Neumann problem) for the case of second-order linear uniformly-elliptic equations — have been studied in greater detail.

In the boundary value problems for elliptic equations, any boundary of the region of the solution may serve as the support of the data. By contrast, in the case of broad classes of equations of hyperbolic and parabolic type non-closed oriented surfaces of the space $ E ^ {n} $ carry the supplementary data, and the domain of definition of the solution substantially depends on these surfaces. These include, for example, the Cauchy problem with initial data and the characteristic Cauchy problem (cf. Cauchy characteristic problem). Boundary value problems for equations of mixed type are posed in a special manner. In the theory of partial differential equations the extensive class of mixed problems has aroused much interest. See Mixed and boundary value problems for hyperbolic equations and systems; Mixed and boundary value problems for parabolic equations and systems.

A problem is considered to be well-posed in the classical sense if it has a unique solution which depends continuously on the data of the problem. Until recently, problems which did not satisfy this requirement were considered meaningless. Since the 1940s, the broad range of mathematical problems in physics, mechanics and technology made it imperative not only to extend the concept of well-posedness of problems involving partial differential equations, but also to extend the meaning of the concept of a solution. So-called generalized solutions were introduced. Beside the question of the existence and uniqueness of exact solutions of problems involving partial differential equations, the concept of approximation of solutions and methods for practical computation have become important in applications.

Historically, the method of separation of variables, or the Fourier method, and the related method of integral transforms (cf. Fourier integral), were among the first methods for the computation of solutions for classes of partial differential equations. Application of this method gave rise to the spectral theory of differential operators.

The parametrix method, which served as the base for the method of potentials (cf. Potentials, method of) was developed more recently. The apparatus of integral equations is applied in this method to the study of boundary value problems of elliptic equations. Methods of the theory of functions of a complex variable, which are successfully employed in the study of elliptic equations with two independent variables, can also be regarded as a major development of the parametrix method. See Differential equation, partial, complex-variable methods.

If the partial differential equation being considered is the Euler equation for a problem of variational calculus in more dimensions, a variational method is often employed. Such a method is very convenient if the Euler equation is of elliptic type. See also Differential equation, partial, variational methods.

Since the 1930s, partial differential equations have been widely investigated by methods of functional analysis, often by the Schauder method and its further development — the method of a priori estimates. The use of these methods permits to establish the existence of weak solutions and strong solutions both for linear and classes of non-linear partial differential equations. See Differential equation, partial, functional methods; Strong solution; Weak solution.

The most popular methods for the computation of approximate solutions of partial differential equations are methods of finite-difference calculus. See also Hyperbolic partial differential equation, numerical methods; Parabolic partial differential equation, numerical methods; Elliptic partial differential equation, numerical methods.

References

[1] L. Bers, F. John, M. Schechter, "Partial differential equations" , Interscience (1964) MR0163043 Zbl 0126.00207
[2] A.V. Bitsadze, "The equations of mathematical physics" , MIR (1980) (Translated from Russian) MR0581247 Zbl 0499.35002
[3] V.S. Vladimirov, "Equations of mathematical physics" , MIR (1984) (Translated from Russian) MR0764399 Zbl 0954.35001 Zbl 0652.35002 Zbl 0695.35001 Zbl 0699.35005 Zbl 0607.35001 Zbl 0506.35001 Zbl 0223.35002 Zbl 0231.35002 Zbl 0207.09101
[4] R. Courant, D. Hilbert, "Methods of mathematical physics. Partial differential equations" , 2 , Interscience (1965) (Translated from German) MR0195654
[5] A.N. Tikhonov, A.A. Samarskii, "Equations of mathematical physics" , Pergamon (1963) (Translated from Russian) MR0165209 Zbl 0111.29008

Comments

Recent years have witnessed a revolution in the theory of partial differential equations. Many important techniques were introduced, such as solution by pseudo-differential or Fourier integral operators, cf. [a8].

References

[a1] P.R. Garabedian, "Partial differential equations" , Wiley (1967) MR1657375 MR0943117 MR0162045 MR0176086 MR0129167 MR0120441 MR0060698 MR0054819 MR0046440 Zbl 0913.35001 Zbl 0124.30501 Zbl 0133.04402 Zbl 0096.06503 Zbl 0058.08902 Zbl 0050.10002
[a2] A.V. Bitsadse, "Equations of mixed type" , Pergamon (1964) (Translated from Russian)
[a3] A.V. Bitsadze, "Boundary value problems for second-order elliptic equations" , North-Holland (1968) (Translated from Russian) MR0226183 Zbl 0167.09401
[a4] F. John, "Partial differential equations" , Springer (1971) MR0304828 Zbl 0209.40001
[a5] V.P. Mikhailov, "Partial differential equations" , MIR (1978) (Translated from Russian) MR0601389 MR0511076 MR0498162 Zbl 0342.35052 Zbl 0111.29009
[a6] A. Friedman, "Partial differential equations of parabolic type" , Prentice-Hall (1964) MR0181836 Zbl 0144.34903
[a7] I.G. Petrovskii, "Ordinary differential equations" , Prentice-Hall (1966) (Translated from Russian) MR0193298
[a8] L.V. Hörmander, "The analysis of linear partial differential operators" , 1–4 , Springer (1983–1985) MR2512677 MR2304165 MR2108588 MR1996773 MR1481433 MR1313500 MR1065993 MR1065136 MR0961959 MR0925821 MR0881605 MR0862624 MR1540773 MR0781537 MR0781536 MR0717035 MR0705278 Zbl 1178.35003 Zbl 1115.35005 Zbl 1062.35004 Zbl 1028.35001 Zbl 0712.35001 Zbl 0687.35002 Zbl 0619.35002 Zbl 0619.35001 Zbl 0612.35001 Zbl 0601.35001 Zbl 0521.35002 Zbl 0521.35001
[a9] D. Gilbarg, N.S. Trudinger, "Elliptic partial differential equations of second order" , Springer (1977) MR0473443 Zbl 0361.35003
How to Cite This Entry:
Differential equation, partial. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Differential_equation,_partial&oldid=33910
This article was adapted from an original article by A.V. Bitsadze (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article