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MathGroup Archive 2000

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Re: Bugs in Abs and Sign !?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25676] Re: Bugs in Abs and Sign !?
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Thu, 19 Oct 2000 04:35:09 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <8sjkr7$g3p@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

Abs[x^2] is simplifyed first to Abs[x]^2 and *than* your rule
Abs[u_]:> fnt[u] is applied. The thame thing happens with
Sign[x^2] that is transformed to Sign[x]^2.

To prevent this simplifications you need Hold[] and 

Hold[Abs[x^2]] /. Abs[u_] :> fnt[u]

works as expected.

Gruss
  Jens

Jack Goldberg wrote:
> 
> 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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