Re: Integrate Bug?

*To*: mathgroup at smc.vnet.net*Subject*: [mg13218] Re: Integrate Bug?*From*: Alan Mahoney <mahoney at purdue.edu>*Date*: Mon, 13 Jul 1998 07:43:07 -0400*Organization*: Purdue University*References*: <6nskhb$fch@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Examining the difference in the terms produced, the "bug" is much easier to reproduce In[1]:= awm = Log[1-W] Out[1]= Log[1 - W] In[2]:= Integrate[D[awm,W],W] Out[2]= Log[-1 + W] But if you do the integration normally, the answer is "Log[Abs[-1 + W]]." Since Mathematica can't know what the value of W (or b Nt W in your case) is, it seems to assume > 1. I would just use /. Log[a_] :> Log[Abs[a_]] before making your numerical substitutions. Alan Ed Hall wrote: > > The following integration problem appears to be a bug in Mathematica's > Integrate function and Wolfram's technical support has been unable to > help so far. I was hoping someone reading this newsgroup might be able > to come up with a solution. > > In[8]:= test = D[F, W] > > In[9]:= F1 = FullSimplify[Integrate[test, W]] > > > In[10]:= N[F /. {b -> 1, Nt -> 2, u -> 2, w -> -1, W -> 0.25, a -> 1, > R -> 1, T -> 1}] > > Out[10]= 0.662394 > > Substituting numeric values into the expression F1 after differentiation > and integration. > > In[11]:= N[F1 /. {b -> 1, Nt -> 2, u -> 2, w -> -1, W -> 0.25, a -> 1, > R -> 1, T -> 1}] > > Out[11]= 0.662394 - 4.06174 I > > The result F1 of the integration is complex whereas the original > expression F before differention is real. How can I insure F1 will be > real and equal to F? > -- Alan W. Mahoney mahoney at purdue.edu 1283 Chemical Engineering Room B5 West Lafayette, IN 47907-1283 765+494-4052