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? -> ->Thanks in advance, ->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 ************************************************************************** * 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 * * Michigan State University * * East Lansing, MI 48824-1122 (USA) * ************************************************************************** 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