       Re: DSolve and K\$31, i am puzzled

• To: mathgroup at smc.vnet.net
• Subject: [mg57099] Re: [mg57078] DSolve and K\$31, i am puzzled
• From: Igor Antonio <igora at wolf-ram.com>
• Date: Sun, 15 May 2005 03:03:59 -0400 (EDT)
• Organization: Wolfram Research, Inc.
• References: <200505140858.EAA09494@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```David Boily wrote:
> When i try to solve this differential equation with the command:
>
> In:= DSolve[f'[t] == Sign[p2 Exp[F2 t] - p1 Exp[F1 t] + p], f[t], t]
>
> where p, p1, p2, F1 and F2 are constants i get the answer
>
>                         F1 t       F2 t
>                   (p - E     p1 + E     p2) (-K\$31 + t)
> Out= {{f[t] -> ------------------------------------- + C}}
>                                F1 t       F2 t    2
>                     Sqrt[(p - E     p1 + E     p2) ]
>
> what the hell is K\$31? I have tried to find out with google or
> the mathematica 5 online docs but have found nothing.
>
> can someone enlighten me?
>
> David Boily
> Centre for Intelligent Machines
> McGill University
> Montreal, Quebec

Those are "module variables" and the numbers give them unique names.
They're needed because often the output from DSolve contains an
unevaluated integral. Example:

In:= DSolve[y'[x]==f[x], y,x]
Out= {{y -> Function[{x}, C + Integrate[f[K\$371], {K\$371, 1, x}]]}}

The K\$371 variable is a dummy variable of integration and does not
affect the calculations.  Just replaced it by another symbol:

In:= %/.{K\$44 -> t}
Out= {{y -> Function[{x}, C + Integrate[f[t], {t, 1, x}]]}}

--
Igor Antonio
Wolfram Research, Inc.
www.wolfram.com

To email me personally, remove the dash.

```

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