Problems in running Dsolve

Question

Sergio Manzetti 2017-12-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/371876-problems-in-running-dsolve

评论： Adarsh Narayan Pandey 2020-11-2

Hi, I have apparently run dsolve in such a way that I got an error in the solution, and now have to re-do the whole thing.

The problem I wanted to solve with dsolve is:

if true
  % code
end
syms h r v(x)
h = 4.5
g = 4/pi
A = pi
expr = [h^2*(diff(v, 2)+2*i*h*diff(v) - g == 0)]
cond = v(0) == A*cos(x);
sol(x) = dsolve(expr,cond)
s = sol(x)

however I get the result:

Error using mupadengine/feval (line 166) Invalid equation or initial condition.

Error in dsolve>mupadDsolve (line 336) T = feval(symengine,'symobj::dsolve',sys,x,options);

Error in dsolve (line 194) sol = mupadDsolve(args, options);

Error in PDE_sol (line 11) sol(x) = dsolve(expr,cond)

any idea what went wrong?

Thanks!

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Sergio Manzetti 2017-12-8

编辑：Sergio Manzetti 2017-12-8

在 MATLAB Online 中打开

PS: Trying another variant,

if true
  % code
end
eqn = h.^2.*diff(y,t, 2) + g.*i.*diff(y, t)== a;
cond = y(0) == 5;
ySol(t) = dsolve(eqn,cond)

and it gives the strangest result:

C29 - (a*t*11150372599265311570767859136324180752990208i)/5959077882784213 + (124330809102446660538845562036705210025114037699336929360115994223289874253133343883264*a*h^2)/35510609213087978610768722029369 - exp(-(t*5959077882784213i)/(11150372599265311570767859136324180752990208*h^2))*((124330809102446660538845562036705210025114037699336929360115994223289874253133343883264*a*h^2)/35510609213087978610768722029369 + C29 - 5)

why is MATLAB making those horrible huge numbers? When I tried such simple eqns in the previous MATLAB I got simple results

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Birdman 2017-12-8

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/371876-problems-in-running-dsolve#answer_295328

编辑：Birdman 2017-12-8

在 MATLAB Online 中打开

v(0) == A*cos(x)

You can not give a symbolic function(cos(x)) as an initial condition. You need something like

v(0)==0

You also need one more initial condition because you have second order differentiation in your equation. Otherwise you will see constants in your solution.

Try this:

syms h r v(x)
h = 4.5
g = 4/pi
A = pi
expr = [h^2*(diff(v, 2)+2*i*h*diff(v) - g == 0)];
Dv=diff(v);
cond = [v(0) == 0,Dv(0)==0];
sol = vpa(dsolve(expr,cond),4)

10 个评论
显示 8更早的评论隐藏 8更早的评论

Pablo Estuardo 2020-2-13

在 MATLAB Online 中打开

A similar error using Dsolve here my friends,..

clear all
close all
syms  u(t) v(t) f taux tauy Hm rho R u0 v0 
du = diff(u);
dv = diff(v);
DE = [du - f*v == taux/(Hm*rho) - R*u;  dv + f*u == tauy/(Hm*rho) - R*v]
Soln = dsolve(DE, u(0)==u0, v(0)==v0)
Error using mupadengine/feval (line 163)
The equations are invalid.
Error in dsolve>mupadDsolve (line 336)
T = feval(symengine,'symobj::dsolve',sys,x,options);
Error in dsolve (line 193)
sol = mupadDsolve(args, options);
 