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''for quadratic forms''

The theorem stating that for any way of reducing a quadratic form (cf. also [[Quadratic forms, reduction of|Quadratic forms, reduction of]])

$$ 
\sum _ { i,j= } 1 ^ { s } 
a _ {ij} x _ {i} x _ {j}  $$

with real coefficients to a sum of squares

$$ 
\sum _ { i= } 1 ^ { s } 
b _ {i} y _ {i}  ^ {2}
$$

by a linear change of variables

$$ 
( x _ {1} \dots x _ {s} )  = \ 
( y _ {1} \dots y _ {s} ) Q ,
$$

where  $  Q $
is a non-singular matrix with real coefficients, the number  $  p $(
respectively,  $  n $) 
of indices  $  i $
for which  $  b _ {i} > 0 $(
or  $  b _ {i} < 0 $) 
is fixed. In its classical form, the law of inertia was established by J.J. Sylvester. This statement is sometimes called Sylvester's theorem.

In its modern form, the law of inertia is the following statement concerning properties of symmetric bilinear forms over ordered fields. Let  $  E $
be a finite-dimensional vector space over an ordered field  $  k $, 
endowed with a non-degenerate symmetric bilinear form  $  f $. 
Then there exists an integer  $  p \geq  0 $
such that for any orthogonal basis  $  e _ {1} \dots e _ {s} $
in  $  E $
with respect to  $  f $
there exist among the  $  s $
elements

$$ 
f ( e _ {i} , e _ {i} ) ,\ \ 
i = 1 \dots s ,
$$

exactly  $  p $
positive and exactly  $  n = s - p $
negative ones. The pair  $  ( p , n) $
is called the signature of  $  f $, 
and the number  $  n $
its index of inertia. Two equivalent forms have the same signature. If  $  k $
is a [[Euclidean field|Euclidean field]], equality of signatures is a sufficient condition for the equivalence of bilinear forms. If the index of inertia  $  n = 0 $, 
the form is called positive definite, and for  $  p = 0 $, 
negative definite. These cases are characterized by the property that  $  f( x, x) > 0 $(
respectively,  $  f ( x , x ) < 0 $) 
for any non-zero  $  x \in E $. 
It follows from the law of inertia that  $  E $
is an orthogonal direct sum (with respect to  $  f  $) 
of subspaces,

$$ 
E  =  E _ {+} \oplus E _ {-} ,
$$

such that the restriction of  $  f $
to  $  E _ {+} $
is positive definite while the restriction of  $  f $
to  $  E _ {-} $
is negative definite and

$$ 
 \mathop{\rm dim}  E _ {+}  =  p ,\   \mathop{\rm dim}  E _ {-}  =  n
$$

(so that the dimensions of  $  E _ {+} $
and  $  E _ {-} $
do not depend on the decomposition).

Sometimes the signature of  $  f $
is taken to be the difference

$$ 
\sigma ( f  )  =  p - n .
$$

If two forms  $  f $
and  $  g $
determine the same element of the [[Witt ring|Witt ring]]  $  W ( k) $
of the field  $  k $, 
then  $  \sigma ( f  ) = \sigma ( g) $. 
Furthermore,  $  \sigma ( f _ {1} \oplus f _ {2} ) = \sigma ( f _ {1} ) + \sigma ( f _ {2} ) $
and  $  \sigma ( f _ {1} \otimes f _ {2} ) = \sigma ( f _ {1} ) \sigma ( f _ {2} ) $
for any non-degenerate forms  $  f _ {1} $
and  $  f _ {2} $, 
and  $  \sigma ( \langle  1 \rangle ) = 1 $, 
so that the mapping  $  f \rightarrow \sigma ( f  ) $
defines a homomorphism from  $  W ( k) $
into the ring of integers  $  \mathbf Z $. 
If  $  k $
is a Euclidean field, then this homomorphism is an isomorphism.

The law of inertia can be generalized to the case of a Hermitian bilinear form over a maximal ordered field  $  k $, 
over a quadratic extension of  $  k $
or over the skew-field of quaternions over  $  k $(
see [[#References|[1]]], [[#References|[4]]]).

====References====
<table><TR><TD valign="top">[1]</TD> <TD valign="top"> N. Bourbaki, "Elements of mathematics. Algebra: Modules. Rings. Forms" , '''2''' , Addison-Wesley (1975) pp. Chapt.4;5;6 (Translated from French) {{MR|2333539}} {{MR|2327161}} {{MR|2325344}} {{MR|2284892}} {{MR|2272929}} {{MR|0928386}} {{MR|0896478}} {{MR|0782297}} {{MR|0782296}} {{MR|0722608}} {{MR|0682756}} {{MR|0643362}} {{MR|0647314}} {{MR|0610795}} {{MR|0583191}} {{MR|0354207}} {{MR|0360549}} {{MR|0237342}} {{MR|0205211}} {{MR|0205210}} {{ZBL|}} </TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top"> S. Lang, "Algebra" , Addison-Wesley (1974) {{MR|0783636}} {{ZBL|0712.00001}} </TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top"> E. Artin, "Geometric algebra" , Interscience (1957) {{MR|1529733}} {{MR|0082463}} {{ZBL|0077.02101}} </TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top"> J. Milnor, D. Husemoller, "Symmetric bilinear forms" , Springer (1973) {{MR|0506372}} {{ZBL|0292.10016}} </TD></TR></table>

====Comments====
The name index is also used for  $  \min ( p, n) $.