       Re: trouble solving partial DE

• To: mathgroup at smc.vnet.net
• Subject: [mg39923] Re: trouble solving partial DE
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Wed, 12 Mar 2003 02:28:07 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <b4k48f\$8ip\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```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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