Question on Positive[x]=True

*To*: mathgroup at smc.vnet.net*Subject*: [mg7791] Question on Positive[x]=True*From*: Raya Firsov-Khanin <raya at mech.ed.ac.uk>*Date*: Tue, 8 Jul 1997 22:41:06 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Dear Steve, I have the following question. I want to state that a variable x is positive. On page 1170 of the Third edition of The Mathematica book it is said that I can do it by making a definition Positive[x] = True However, while trying to do it I get a message : Set::write: Tag Positive in Positive[x] is Protected Any suggestions on how one can make variables positive, negative etc? Thanks a lot. Raya Khanin. University of Edinburgh