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Re: trouble solving partial DE


Hi,

for what is the first argument in u[x,t] ??
You have only derivatives with respect to t
and it is an ordinary differential equation for
u[t]. But you gave not an intial condition
u[x,0]==0 *and* Derivative[0,1][u][x,0]==blubBlub

you gave D[u[L, 0], t] == 0 and this evaluates 
to True, and Mathematica miss a second initial condition.


Regards
  Jens


karlo jolic wrote:
> 
> Hi everyone
> 
> I keeping getting the following error:
> 
> NDSolve::"deql": "The first argument must have both an equation and an \
> 
> initial condition."
> 
> when i try to solve the following second order partial differential
> equation
> (wherein the function u is a function of variables x and t):
> 
> NDSolve[{m*D[u[x, t], t, t] + 6*Pi*n*r*D[u[x, t], t] -
>          0.5*mu*i*ndash*pm*((L - u[x, t])^2/(R^2 + (L - u[x, t])^2)^1.5
> -
>          1/(R^2 + (L - u[x, t])^2)^0.5 - (L + u[x, t])^2/(R^2 + (L +
> u[x, t])^2)^1.5 +
>          1/(R^2 + (L + u[x, t])^2)^0.5) == 0,
>          u[L, 0] == L,
>          D[u[L, 0], t] == 0},
>          u, {x, 0, L}, {t, 0, 1}]
> 
> I have specified 2 initial conditions already, so i can't understand why
> i get the error.
> 
> does anyone know whats the problem?
> 
> Thanks
> 
> Karlo
> PS I'm relatively new at mathematica.


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