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Cannot Factor an expression

• To: mathgroup at smc.vnet.net
• Subject: [mg87563] Cannot Factor an expression
• From: Francogrex <franco at grex.org>
• Date: Sun, 13 Apr 2008 03:31:17 -0400 (EDT)

If Factor is effectively the reverse of expand, then how come factor
cannot find back the expression below?

Expand[Sum[(Subscript[z, i] - 1)*(\[Mu] - Subscript[x, i])^2, {i, 1,
3}]]

Factor[-3*\[Mu]^2 + 2*\[Mu]*Subscript[x, 1] - Subscript[x, 1]^2 +
   2*\[Mu]*Subscript[x, 2] - Subscript[x, 2]^2 + 2*\[Mu]*Subscript[x,
3] -
   Subscript[x, 3]^2 + \[Mu]^2*Subscript[z, 1] -
   2*\[Mu]*Subscript[x, 1]*Subscript[z, 1] + Subscript[x, 1]^2*
    Subscript[z, 1] + \[Mu]^2*Subscript[z, 2] -
   2*\[Mu]*Subscript[x, 2]*Subscript[z, 2] + Subscript[x, 2]^2*
    Subscript[z, 2] + \[Mu]^2*Subscript[z, 3] -
   2*\[Mu]*Subscript[x, 3]*Subscript[z, 3] + Subscript[x, 3]^2*
    Subscript[z, 3]]

Is there some additional arguments to the function factor so that it
can find back the original:
Sum[(Subscript[z, i] - 1)*(\[Mu] - Subscript[x, i])^2, {i, 1, 3}]

Thanks for help with this

• Follow-Ups:
  • Re: Cannot Factor an expression
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>