0 个投票

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!

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Sergio Manzetti
Sergio Manzetti 2017-12-8
编辑：Sergio Manzetti 2017-12-8
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

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 采纳的回答

Birdman
Birdman 2017-12-8
编辑：Birdman 2017-12-8

1 个投票

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)

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Sergio Manzetti
Sergio Manzetti 2017-12-8
Birdman, thanks. Of course, I was a little fast here...Cheers
Birdman
Birdman 2017-12-8
You are welcome Sergio. If my answer helped, can you accept it?
Sergio Manzetti
Sergio Manzetti 2017-12-8
PS: What is 4 in :
sol = vpa(dsolve(expr,cond),4) ?
Sergio Manzetti
Sergio Manzetti 2017-12-8
Birdman, the initial condition of being y_0= cos(x) is vital. It is allowed in PDEs, however I must eventually change the system to a PDE in case of using cos(x) as initial condition?
Birdman
Birdman 2017-12-8
As far as I know, there is a function pdepe, that should do it for you.
But I assume you need cos(x) function's value at a certain point, right?
Birdman
Birdman 2017-12-8
Btw, 4 is for displaying four digits after decimal.
Sergio Manzetti
Sergio Manzetti 2017-12-8
THanks Birdman. All the best
Pablo Estuardo
Pablo Estuardo 2020-2-13
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);
I will appreciate any suggestion
Hey, did you find any solution to this error?

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