user-defined function as input in "dsolve"

Hello everyone, I am new with MATLAB so maybe I can do the following and still haven't figured out how.

I want to use

dsolve ('Dz=a*z+b*t+c(z,t)','t')

where a and/or b are symbolic and

c(z,t) is a user-defined function?

The specific form of the expression is only an example (in principle it could/should be more general. The important thing here is the use of a user-defined function within the expression that defines the differential equation to be solved.

Is there any way I ca do something to this effect?

Answer 1

[Miltos]

I think I found the way - it was hidden in an example about using dsolve for a system of ODE that are extracted from a matrix!!

Anyway...

So suppose you have a defined a function r(x,y) (either directly with inline or anonymous function, or extracting from a solution of a previous system, matrix etc)

Then as I understand it you create the string of it

eq=char(r(x,y))

[note if r(x,y) corresponds to a solution ans.w of a previous system then one can define the string eq diectly as eq=char(ans.w)]

and then you use dsolve as follows

dsolve(strcat('Dx=',eq),'t')

[where t is the independent variable (which could be y)]

stracat(s1,s2) is a built-in function that "keeps together" the two strings (in general more than two) 'Dx=' and eq creating a new string that corresponds to the symbolic expression implied by Dx=r(x,y).

It has worked for few examples I played with, but maybe there is a flaw in this I cannot see, or maybe there is a more efficient way of doing it.

[Luan Trinh]

Hi everyone,

I face a problem similar to this. Could you please help?

-------------------------------------------------------------

r_si; m_si (r_si and m_si are extract from the previous function)

f_r = char(r_si); f_m = char (m_si); f_tot = f_r^2/(E*t^3/12)*f_m;

inits = 'Dy(0)=0, Dy(pi/2)=0';

u = dsolve(strcat('D2y+y=',f_r^2/f_m),inits,'si');

----------------------- This is error message.

In an assignment A(:) = B, the number of elements in A and B must be the same.

Error in strcat (line 94) s(pos:pos+len-1) = str;

Error in Moment_curvature (line 25) y = dsolve(strcat('D2y + y ==',f_tot),inits,'si');