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

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

```

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