       Re: How to simplify?

• To: mathgroup at smc.vnet.net
• Subject: [mg96204] Re: How to simplify?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Mon, 9 Feb 2009 05:34:56 -0500 (EST)
• References: <gmndih\$s09\$1@smc.vnet.net>

```Hi,

you have a ode of second order and it should have
to constants, I would expect that

{C->0,C->1} will help ..

or

DSolve[{y''[r] + 1/r y'[r] + S[r] == BesselJ[0, r], y == 0}, y[r],
r] // FullSimplify

simply add an initial condition ??

Regards
Jens

Aaron Fude wrote:
> Hi,
>
> I'm sorry for totally belaboring this point, but I am having a hard
> time getting Mathematica be useful for me in this one respect. The
> following code shows that the linear ODE that I am trying to solve has
> 1/2 r BesselJ[1, r] as the particular solution. DSolve, however,
> returns an answer that I'm sure is correct. I tested it - numerically,
> the particular part is exactly 1/2 r BesselJ[1, r].
>
> But for someone who is looking for analytical insight, the answer is
> not useful. What can be done to simplify the expression so that it
> appears as 1/2 r BesselJ[1, r]
>
>
> S[r_] := 1/2 r BesselJ[1, r];
> D[S[r], {r, 2}] + 1/r D[S[r], r] + S[r] // FullSimplify
> DSolve[y''[r] + 1/r y'[r] + S[r] == BesselJ[0, r], y[r],
>   r] // FullSimplify
>
> I will gladly accept the answer "Nothing" and move on to looking for
> alternative solutions. Also note, this is just a model problem for me
> in preparation for more complicated ones. Last time I got several
> responses that said - "if you already know the solution to this
> problem, why are you trying to solve it?"
>