       Re: diff equation

• To: mathgroup at smc.vnet.net
• Subject: [mg88333] Re: diff equation
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Fri, 2 May 2008 03:42:12 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <BLU129-DS335A7C6B4ED0F214741F6A8D90@phx.gbl> <4818FDAB.60005@gmail.com> <fvc5v1\$9ks\$1@smc.vnet.net>

```Ali YILDIRIM wrote:

> [resent a more readable form - moderator]
>
>  Dear Math group..
>
>  I have an differential equation. I can not solve in mathematica.
>  Would you help me, please?
>
>   dM/d t =D[d^2 M//dr^2 +(2/r) (dM/dr)]-kM
-----------!!!----!!!!
Not sure for what this D stands here. I discarded it in the example
below. Also you have a spurious backslash. So you could write your
equation as

D[M[r, t], t] == D[M[r, t], r, r] + 2/r D[M[r, t], r] - k M[r, t]

>   Boundary conditions are:
>
>  1-) at t=0,  M= Mo

M[r, 0] == M0

>  2-) r = 0,   dM/dr=0

D[M[0, t], t] == 0

>  3-) at r = R,   M = Ms

M[R, t] == Ms

Then you can use DSolve or NDSolve as in

DSolve[{D[M[r, t], t] ==
D[M[r, t], r, r] + 2/r D[M[r, t], r] - k M[r, t], M[r, 0] == M0,
D[M[0, t], t] == 0, M[R, t] == Ms}, M, {r, t}]

Regards,
-- Jean-Marc

```

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