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RE: Bugs in Abs and Sign !?

  • To: mathgroup at
  • Subject: [mg25701] RE: [mg25674] Bugs in Abs and Sign !?
  • From: "David Park" <djmp at>
  • Date: Thu, 19 Oct 2000 04:35:27 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

Dear Jack,

I don't think it is a bug. You get that result because Mathematica
simplifies the expression before the rule is applied.

{Abs[x^2], Sign[x^2]}
{Abs[x]^2, Sign[x]^2}

{Abs[x^n], Sign[x^n]}
FullSimplify[%, n \[Element] Integers]
{Abs[x^n], Sign[x^n]}
{Abs[x]^n, Sign[x^n]}

David Park
djmp at

> -----Original Message-----
> From: Jack Goldberg [mailto:jackgold at]
To: mathgroup at
> Hi group.
> Try,
> Abs[x^2] /. Abs[u_] -> fnt[u]
> and
> Sign[x^2] /. Sign[u_] -> fnt[u]
> In each case I get  fnt[x]^2.  Shouldn't I get  fnt[x^2]? After all,
> g[x^2] /. g[u_] -> fnt[u]  returns fnt[x^2] which is what I have been led
> to expect from Mathematica.  I am using ver 4.01 on a PowerMac.
> These peculiarities are driving me crazy.  This is the fourth "bug" I have
> found in just 2 weeks.  Please tell me I'm wrong.
> Thanks guys.
> Jack

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