dsolve invalid indexing error

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Joe Dombroski
Joe Dombroski 2022-4-26
回答： Joe Dombroski 2022-4-28
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)
  1 个评论
Torsten
Torsten 2022-4-26
编辑：Torsten 2022-4-26
What kind of solution for y do you expect for an unspecified function u(t) ?
That's where the error comes from, I guess.

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采纳的回答

Walter Roberson
Walter Roberson 2022-4-26
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.

更多回答（1 个）

Joe Dombroski
Joe Dombroski 2022-4-28
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)

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