The RK4 function uses the fourth-order Runge-Kutta method to advance a solution to a system of ordinary differential equations one time-step H, given values for the variables Y and their derivatives Dydx known at X.
RK4 is based on the routine rk4 described in section 16.1 of Numerical Recipes in C: The Art of Scientific Computing (Second Edition), published by Cambridge University Press, and is used by permission.
To integrate the example system of differential equations for one time step, H:
; Define the step size:
H = 0.5
; Define an initial X value:
X = 0.0
; Define initial Y values:
Y = [4.0, 6.0]
; Calculate the initial derivative values:
dydx = DIFFERENTIAL(X,Y)
; Integrate over the interval (0, 0.5):
result = RK4(Y, dydx, X, H, 'differential')
; Print the result:
This is the exact solution vector to five-decimal precision.
Result = RK4( Y, Dydx, X, H, Derivs [, /DOUBLE] )
Returns the integrations of the ordinary differential equations.
A vector of values for Y at X
Note: If RK4 is complex then only the real part is used for the computation.
A vector of derivatives for Y at X.
A scalar value for the initial condition.
A scalar value giving interval length or step size.
A scalar string specifying the name of a user-supplied IDL function that calculates the values of the derivatives Dydx at X. This function must accept two arguments: A scalar floating value X, and one n-element vector Y. It must return an n-element vector result.
For example, suppose the values of the derivatives are defined by the following relations:
dy0 / dx = –0.5y0, dy1 / dx = 4.0 – 0.3y1 – 0.1y0
We can write a function DIFFERENTIAL to express these relationships in the IDL language:
FUNCTION differential, X, Y
RETURN, [-0.5 * Y, 4.0 - 0.3 * Y - 0.1 * Y]
Set this keyword to force the computation to be done in double-precision arithmetic.
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