Error using ode45. Must return a column vector?

Hi everyone!

I want to call ode45 like that:

[td, xp] = ode45(@diffeqns, [t, tfin], xp);

Where diffeqns is defined as:

function dxp = diffeqns(t,xp)
    global GM
    GM = 6.6742e-11 * 5.9736e24 * 10^-9 * 3600^2;
    global Q
    Q = [0 0 0 0; 0 0 0 0; 0 0 0.001 0; 0 0 0 0.001];
    r = sqrt(xp(1)^2+xp(2)^2);
    
    x = xp(1:4);
    dx(1) = x(3);
    dx(2) = x(4);
    dx(3) = GM*x(1)/r^3;
    dx(4) = GM*x(2)/r^3;
    
    F = [0 0 1 0; 0 0 0 1; (GM*3*x(1)^2-r^2)/r^5 GM*3*x(1)*x(2)/r^5 0 0; ...
        GM*3*x(1)*x(2)/r^5 GM*3*x(2)^2-r^2/r^5 0 0];
    P = xp(5:20);
    P = reshape(P,4,4);
    dp = F*P+P*F'+Q;
    dp = reshape(dp,1,16);
    
    dxp = [dx dp];
    
end

dxp is a 1x20 vector, but I still get the error:

Error using odearguments (line 90)

DIFFEQNS must return a column vector.

Does anybody has any suggestion?

Thanks

Answer 1

[Stephan]

1x20 is not a column vector, but a row vector. transpose it to 20x1.