function z = VTB1_3(rkf,u,t,x0)%VTB1_3   VTB1_3(rkf,u,t,x0)%       Runge-Kutta fourth order solution to a first order DE%       t is a row vector from the initial time to the final time%       step h.  x0 is the initial value of the function.%       The force matrix u is ordered such that the nth column%       of u is the force vector u evaluated at time n*dt.%       rkf is a variable containing the name of the function %       file.  The equations in the function file must be written%       in first order state space form.%       See example VTB1_3ex.m.%       Example%       t=0:.5:20;  % Creates time vector%       u=[zeros(1,length(t));sin(t*1.1)];% Creates force matrix%       x0=[1 ;0];  % Creates initial state vector.%       x=vtb1_3('vtb1_3ex',u,t,x0); % Runs analysis.%       plot(t,x(1,:)); % Plots displacement versus time.%       plot(t,x(2,:)); % Plots velocity versus time.n=length(t);z=zeros(length(x0),length(t));z(:,1)=x0;h=t(2)-t(1);for l1=1:(n-1);   z1=z(:,l1);   u1=u(:,l1);   u2=u(:,l1+1);   k1=h*feval(rkf,z1,u1,t(l1));   k2=h*feval(rkf,z1+.5*k1,u1,t(l1)+.5*h);   k3=h*feval(rkf,z1+.5*k2,u1,t(l1)+.5*h);   k4=h*feval(rkf,z1+k3,u1,t(l1)+h);   z(:,l1+1)=z(:,l1)+1/6*(k1+2*k2+2*k3+k4);end