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MathGroup Archive 2000

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Re: Bugs in Abs and Sign !?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25696] Re: Bugs in Abs and Sign !?
  • From: Brian Higgins <bghiggins at ucdavis.edu>
  • Date: Thu, 19 Oct 2000 04:35:23 -0400 (EDT)
  • References: <8sjkr7$g3p@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Jack, There is no bug in my opinion. Mathematica is doing pricesly
what you requested. When you use replacement rules it is wise to
carefully examine the FullForm  of your expression. Thus

FullForm[Abs[x^2] ]

Power[Abs[x], 2]

Hence

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

will give

Power[ fnt[x] , 2]

which is

fnt[x]^2

The correct replacement rule for what you want to do is

Abs[x^2] /. Abs -> fnt

Cheers,

Brian



In article <8sjkr7$g3p at smc.vnet.net>,
  Jack Goldberg <jackgold at umich.edu> 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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