dsolve invalid indexing error

I have a simple diff eqn that is throwing a dsolve error.

Invalid indexing or function definition. Indexing must follow MATLAB indexing.

Function arguments must be symbolic variables, and function body must be sym

expression.

I am guessing it is a simple syntax error, but I just can't see it.

syms y(t) u(t)

Dy = diff(y,t);

D2y = diff(Dy,t);

D3y = diff(D2y,t);

eqn = D3y + 7*D2y + 20*Dy + 24*y == 3*diff(u,t) + 2*u;

cond = [y(0)==0, Dy(0)==1, D2y(0)==-1];

ySol(t) = dsolve(eqn,cond)

[Torsten]

What kind of solution for y do you expect for an unspecified function u(t) ?

That's where the error comes from, I guess.

Answer 1

[Walter Roberson]

You have two functions, y and u, but only one differential equation. MATLAB has to decide what to solve for. It essentially uses symvar() and picks the first one... which would be u. u only appears as first derivative, while you have 3 boundary conditions.

The error you get occurs when you have more boundary conditions than you have differential equations.

Answer 2

[Joe Dombroski]

This runs, I just needs to work on how to generate the state equations. Thanks all for the sage advice!

syms y(t) u(t)

Dy = diff(y,t);

D2y = diff(Dy,t);

D3y = diff(D2y,t);

yeqn = D3y + 7*D2y + 20*Dy + 24*y == 0;

ueqn = 3*diff(u,t) + 2*u ==0;

cond = [y(0)==0, Dy(0)==1, D2y(0)==-1];

ySol = dsolve(yeqn,cond)

uSol = dsolve(ueqn)