Error using mupadengine/feval (line 187) Invalid equations.

Question

Lada Nuzhna 2019-12-28

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/498318-error-using-mupadengine-feval-line-187-invalid-equations

评论： Lada Nuzhna 2020-1-1

采纳的回答： Jyothis Gireesh

在 MATLAB Online 中打开

I am trying to solve a differential equation, but getting the following error

Error using mupadengine/feval (line 187)

Invalid equations.

My code:

syms r(P);
eqn = diff(r,P,2) == (r^4 / b^2) - (1-k/r)*(r^4/a^2 + r^2);
Dr = diff(r,P);
cond = [ r(0) == 0.1, Dr(0) == 0.1];
rsol(P) = dsolve(eqn ,[0 10], cond);

All the variables used (a,k,b) are defined in the code before.

Will be grateful for any help!

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Answer 1

Jyothis Gireesh 2019-12-31

1
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/498318-error-using-mupadengine-feval-line-187-invalid-equations#answer_408238

在 MATLAB Online 中打开

I am assuming here that you want the solution to be evaluated over the range [0 10]. But according to MATLAB documentation, the "dsolve()" doesn’t provide the functionality to provide that as an input argument to the function.

I tried running the above code by initializing the values of "a", "b" and "k" with value 1 which didn't yield an explicit solution. In case such a situation arises, it may be better to use numerical integration techniques as ode45. The following code may be useful in case of numerical methods

clear; 
syms r(p) 
a=2; 
b=1; 
k=1; 
eqn = diff(r,p,2) == r^4/b^2 - (1-k/r)*(r^4/a^2 + r^2); 
Dr = diff(r,p); 
V = odeToVectorField(eqn); 
F = matlabFunction(V, 'vars', {'p','Y'}); 
rsol = ode45(F,[0 10], [0.1 0.1]) 