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MathGroup Archive 2000

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RE: rules for Sign[_]^n

  • To: mathgroup at smc.vnet.net
  • Subject: [mg26355] RE: [mg26337] rules for Sign[_]^n
  • From: "Ingolf Dahl" <f9aid at fy.chalmers.se>
  • Date: Wed, 13 Dec 2000 02:41:16 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi!
I would recommend that you add Real to the specification of your variables,
otherwise you might get problems with complex numbers:

{(Sign[_Real])^(y_?EvenQ) :> 1, (Sign[x_Real])^(y_?OddQ) :> Sign[x]}

You know, Sign[I]^2 == -1.

This works if you apply the rule to numbers, which is not very useful, since
you then are able to calculate Sign anyway. Probably you want to apply the
rules to symbolic variables that are known to be real, and then the above
approach will not work. In that case, you can define z to be real by

z /: Im[z] = 0;
(see Help for Add-on Standard Package Algebra`ReIm`, but this package does
not seem to be necessary)

Then the following rule seems to work

{(Sign[x_ /; Im[x] == 0])^(y_?EvenQ) :>
    1, (Sign[x_ /; Im[x] == 0])^(y_?OddQ) :> Sign[x]}

If you want to teach Mathematica to apply this rule, you have to modify
Power:

Unprotect[Power];
Power[Sign[x_ /; Im[x] == 0], y_?EvenQ] := 1;
Power[Sign[x_ /; Im[x] == 0], y_?OddQ] := Sign[x];
Protect[Power];

Maybe the package Algebra`ReIm` could be useful for you, check the help.

I hope that I have got everything right

Kind regards

Ingolf Dahl
Chalmers University


> Adalbert Hanssen [hanssen at Zeiss.de] wrote in  [mg26337] rules for
Sign[_]^n:
Hi, MathGroup,

dealing with Sign, it would be useful, if Mathematica
knew

{(Sign[_])^(y_?EvenQ):>1, (Sign[x_])^(y_?OddQ):>Sign[x]}

How can I teach Mathematica this rule (in Init.m), such that it
automaticly applies it in Simplify, Expand and the like
whenever applicable?

kind regards

Dipl.-Math. Adalbert Hanszen



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