Matlab dsolve issue with simple equations

I am trying to solve simple equations as:

syms  e1(t) e2(t) f1(t) f2(t) R1 C1
Vc_dot=diff(e2);
cond=[Vc_dot(0)==0]

cond = 

eqns = [e1 == e2; f2 == C1*Vc_dot; f1 == f2]

eqns(t) = 

S=dsolve(eqns,cond)

Error using symengine
Invalid equation or initial condition.

Error in mupadengine/evalin_internal

Error in mupadengine/fevalHelper

Error in mupadengine/feval_internal

Error in dsolve>mupadDsolve (line 334)
T = feval_internal(symengine,'symobj::dsolve',sys,x,options);

Error in dsolve (line 203)
sol = mupadDsolve(args, options);

I am getting this error although equations are trivial

[Torsten]

"dsolve" accepts differential systems of equations, not differential-algebraic systems as in your case.

And I'm not sure what you expect as solution.

Answer 1

[SAI SRUJAN]

Hi Ahmed Khamis,

I can understand that you are facing an issue using the "dsolve" MATLAB function.

You encountered this error as "dsolve" only accepts differential system of equations. Line 4 of your code is causing the error as the equations are not all differential system of equations.

You can follow the below given example to resolve the issue.

syms e(t) e1(t) e2(t) c

eqn1=[diff(e,t)==e*c];
S=dsolve(eqn1);

The above code will exceute as the "eqn1" contains only differential set of equations.

eqn2=[e1==e2 ,diff(e,t)==e*c];
S=dsolve(eqn2);

The above code snippet errors out as the all the equations in "eqn2" are not differential set of equations.

You can refer to the below documentation to understand more about "dsolve" function in MATLAB.

Solve system of differential equations - MATLAB dsolve - MathWorks India