Possible Collect bug

*To*: mathgroup at yoda.ncsa.uiuc.edu*Subject*: Possible Collect bug*From*: John Mitchell <uunet!pico.uchicago.edu!mitchell>*Date*: Mon, 20 Aug 90 17:48:34 CDT

I would like to relate a recent calculation I ran across which I believe represents a bug in Mathematica 1.2.1 (Macintosh II version). I have not seen this bug reported elsewhere, but it may have been. The sample ouput below shows a comparison of Collect with Factor on what seems like a straightforward expression. Note, however, that two different answers result. I would appreciate anyone's comments on this. (Note that this output has been "cut and pasted" from Mathematica into a word-processing program before transfer, explaining the missning In[##] and Out[##] ) expr = -((-1)^(2n) y^2)/8 - ((-1)^(-1 + 2n) y^2)/8 + ((-1)^(1 + 2n) y^2)/8 2 n 2 -1 + 2 n 2 1 + 2 n 2 -((-1) y ) (-1) y (-1) y ------------- - ----------------- + ----------------- 8 8 8 Collect[expr,y] 2 n 2 -3 (-1) y ------------- 8 (* This is incorrect *) Factor[expr] 2 n 2 -((-1) y ) ------------- 8 (* This is correct *) ***************************** John Mitchell Univ. of Chicago 5735 South Ellis Ave. Chicago, IL 60637 (312) 702-7253 mitchell at pico.uchicago.edu