Problem with Collect

*To*: mathgroup at yoda.ncsa.uiuc.edu*Subject*: Problem with Collect*From*: Markus Lautenbacher <Markus.Lautenbacher at physik.tu-muenchen.de>*Date*: Fri, 1 Feb 91 23:27:04 -0600

Colin Williams <cwilliam at parc.xerox.com> noticed an unexpected behavior of Collect[]. I have no explanation for this. Instead I want to bring another point to your attention which I found when experimenting with Collect: (* the original example *) In[1]:= t = 2^(n-3) + x -3 + n Out[1]= 2 + x (* something that should be equivalent to the original example *) In[2]:= texp = t // ExpandAll n 2 Out[2]= -- + x 8 In[3]:= t - texp // Simplify Out[3]= 0 In[4]:= Collect[t,x] n Out[4]= 2 + x In[5]:= Collect[texp,x] n 2 Out[5]= -- + x 8 So a temporary cure to the problem seems to be to use "Collect[ExpandAll[expr],x]" instead of "Collect[expr,x]". Anyway it is not that unreasonable to expand everything (to "bring the x's out") before passing it to Collect, although I expected Collect to do the expansion automatically. MARKUS