MATLAB: Solving a differential equation with ODE45

Question

Hello,

I have 4 equations and I would like to solve it with a ode45 solver, however it does not work. Normally I use this solver to for solving equations with 2 variables.

Thanks for help!

ERROR:

Error using vertcat

Dimensions of arrays being concatenated are not consistent.

Error in exponentialansatz_dgl (line 16)

xdot= [ x(3);

Error in odearguments (line 90)

f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)

odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in exponentialansatz (line 68)

[t,y]=ode45(@exponentialansatz_dgl,tspan,x0);

CODE

main program

tspan = [0 20];

x1 = 2;

x2 = 3;

x3 = 0;

x4 = 1;

x0 = [ x1;

x2;

x3;

x4];

[t,y]=ode45(@exponentialansatz_dgl,tspan,x0);

function

function xdot = exponentialansatz_dgl(t,x)

x1=x(1);

x2=x(2);

x3=x(3);

x4=x(4);

k1 = 1;

k2 = 2;

k3 = 1.5;

m1= 2;

m2 = 1;

xdot= [ x(3);

x(4);

k2/m1 -(k1+k2)/m1;

k2/m2 -(k2+k3)/m2; ]

Answer 1

Remove the space in the middle of each of the last two lines of the xdot matrix. You probably also want to end the definition of xdot with a semicolon to prevent MATLAB from displaying xdot each time ode45 calls this function.

 xdot= [ x(3);
         x(4);
         k2/m1-(k1+k2)/m1;
         k2/m2-(k2+k3)/m2; ];