Re: How to set Sign of expression?

• To: mathgroup at christensen.cybernetics.net
• Subject: [mg1650] Re: How to set Sign of expression?
• From: rubin at msu.edu (Paul A. Rubin)
• Date: Sat, 8 Jul 1995 05:15:34 -0400
• Organization: Michigan State University

```In article <3t2io7\$mie at news0.cybernetics.net>,
keeton at husc.harvard.edu (Charles Keeton) wrote:
->Hi.  I want to set the Sign of an expression.  For example, suppose
->I know a is restricted to the range 0 < a < 1; then I know 1-a > 0.
->How do I tell Mathematica this?  I have tried
->
->  Sign[1-a]^=1
->
->and
->
->  a /: Sign[1-a]=1
->
->but Mathematica frowns on both.  Any ideas?
->
->Chuck

Been there; done that; doesn't work.  The two problems seem to be that (a)
you can't count on Mma checking whether a<1 when you want it to and (b) you
can't predict how it will do the check if indeed it does.  For instance,
even with your up-value (the right approach IMHO), I don't think Less[a,2]
would return True.  (BTW, you might try a /: Less[a,1] = True.)

Paul

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* Paul A. Rubin                                  Phone: (517) 432-3509   *
* Department of Management                       Fax:   (517) 432-1111   *
* Eli Broad Graduate School of Management        Net:   RUBIN at MSU.EDU    *
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Mathematicians are like Frenchmen:  whenever you say something to them,
they translate it into their own language, and at once it is something
entirely different.                                    J. W. v. GOETHE

```

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