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Possible Collect bug II


The following are the findings of John Mitchell reporting to mathgroup
August 21.


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

***********************************

I tried a few things and found this:

FixedPoint[Simplify, (expr - Simplify[expr])]

0

FixedPoint[Simplify, (expr - Collect[expr,y])]

      2 n   2
  (-1)    y
 ------------
      4  
***********************************
Howard Fink
fink at acf5.nyu.edu



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