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>*Reply-to*: igora at wolf-ram.com*Sender*: owner-wri-mathgroup at wolfram.com

David Boily wrote: > When i try to solve this differential equation with the command: > > In[1]:= 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[1]= {{f[t] -> ------------------------------------- + C[1]}} > 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[1]:= DSolve[y'[x]==f[x], y,x] Out[1]= {{y -> Function[{x}, C[1] + 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[2]:= %/.{K$44 -> t} Out[2]= {{y -> Function[{x}, C[1] + Integrate[f[t], {t, 1, x}]]}} -- Igor Antonio Wolfram Research, Inc. www.wolfram.com To email me personally, remove the dash.

**Follow-Ups**:**Re: Re: DSolve and K$31, i am puzzled***From:*Igor Antonio <igora@wolf-ram.com>

**References**:**DSolve and K$31, i am puzzled***From:*David Boily <dsboily@fastmail.ca>