Adaptive Runge–Kutta method
ARK method
An s - stage adaptive Runge–Kutta method for the computation of approximations u _ {m} for the solution y ( t _ {m} ) of an initial-value problem
y ^ \prime = f ( t,y ) , y ( t _ {0} ) = y _ {0} , t \in [ t _ {0} ,t _ {e} ] ,
is given by
u _ {m + 1 } ^ {( 1 ) } = u _ {m} ,
u _ {m + 1 } ^ {( i ) } = R _ {0} ^ {( i ) } ( c _ {i} hT ) u _ {m} +
+ h \sum _ {j = 1 } ^ { {i } - 1 } A _ {ij } ( hT ) \left [ f ( t _ {m} + c _ {j} h,u _ {m + 1 } ^ {( j ) } ) - Tu _ {m + 1 } ^ {( j ) } \right ] ,
i = 2 \dots s,
u _ {m + 1 } = R _ {0} ^ {( s + 1 ) } ( hT ) u _ {m} +
+ h \sum _ {j = 1 } ^ { s } B _ {j} ( hT ) \left [ f ( t _ {m} + c _ {j} h,u _ {m + 1 } ^ {( j ) } ) - Tu _ {m + 1 } ^ {( j ) } \right ] .
Here, T is an arbitrary matrix, for stability reasons usually T \approx f _ {y} ( t _ {m} ,u _ {m} ) . For T = 0 the method reduces to an explicit Runge–Kutta method. The c _ {i} are real parameters and R _ {0} ^ {( i ) } ( z ) , z \in \mathbf C , are rational approximations to e ^ {z} for z \rightarrow 0 . The rational matrix functions A _ {ij } , B _ {j} are defined by
A _ {ij } ( z ) = \sum _ {l = 0 } ^ { \rho _ {i} } R _ {l + 1 } ^ {( i ) } ( c _ {i} z ) c _ {i} ^ {l + 1 } \lambda _ {lj } ^ {( i ) } ,
B _ {j} ( z ) = \sum _ {l = 0 } ^ { \rho _ {s + 1 } } R _ {l + 1 } ^ {( s + 1 ) } ( z ) \lambda _ {lj } ^ {( s + 1 ) } ,
with \lambda _ {lj } ^ {( i ) } \in \mathbf R and
R _ {1} ^ {( i ) } ( z ) = { \frac{R _ {0} ^ {( i ) } ( z ) - 1 }{z} } ,
R _ {l + 1 } ^ {( i ) } ( z ) = { \frac{lR _ {l} ^ {( i ) } ( z ) - 1 }{z} } .
The computation of u _ {m + 1 } requires the solution of linear systems of algebraic equations only. The coefficients \lambda _ {lj } ^ {( i ) } are determined to give a high order of consistency or B-consistency ([a2]). Applied to the test equation of A-stability, y ^ \prime = \lambda y with { \mathop{\rm Re} } \lambda \leq 0 , an adaptive Runge–Kutta method with T = \lambda yields
u _ {m + 1 } = R _ {0} ^ {( s + 1 ) } ( h \lambda ) u _ {m} .
By the corresponding choice of stability functions R _ {0} ^ {( s + 1 ) } ( z ) , adaptive Runge–Kutta methods are A- or L-stable and therefore well suited for stiff systems (cf. Stiff differential system). Furthermore, they can be easily adapted to the numerical solution of partitioned systems, where only a subsystem of dimension n _ {s} < n is stiff. Here, by a corresponding choice of T the dimension of the linear systems to be solved can be reduced to n _ {s} [a1].
References
[a1] | K. Strehmel, R. Weiner, "Partitioned adaptive Runge–Kutta methods and their stability" Numer. Math. , 45 (1984) pp. 283–300 |
[a2] | K. Strehmel, R. Weiner, "B-convergence results for linearly implicit one step methods" BIT , 27 (1987) pp. 264–281 |
Adaptive Runge–Kutta method. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Adaptive_Runge%E2%80%93Kutta_method&oldid=55403