package odeadapt; import Jama.*; /** * ODESolverRK4.java * solves a given ODE by giving the "next" value for x * using Runge-Kutta method of order 4 */ public class ODESolverRK4 extends ODESingleStepSolver { /** * construct solver for a given ODE */ public ODESolverRK4(ODE ode) { super(ode); order = 4; } /* * integrate until t + h * using a fourth order Runge-Kutta scheme */ public int nextStep(double h) { Matrix k1 = ode.f(x, t); double t1 = t + h / 2.0; // x1 = x + h*k1/2.0; Matrix x1 = x.plus(k1.times(h / 2.0)); Matrix k2 = ode.f(x1, t1); double t2 = t1; // x2 = x + h*k2/2.0; Matrix x2 = x.plus(k2.times(h / 2.0)); Matrix k3 = ode.f(x2, t2); double t3 = t + h; // x3 = x + k3*h; Matrix x3 = x.plus(k3.times(h)); Matrix k4 = ode.f(x3, t3); // x += (k1 + 2.0*k2 + 2.0*k3 + k4)*h/6.0; x.plusEquals(k1.times(h / 6.0)); x.plusEquals(k2.times(h / 3.0)); x.plusEquals(k3.times(h / 3.0)); x.plusEquals(k4.times(h / 6.0)); t += h; return 1; } }