How to use sign[] for symbolic operations?

*To*: mathgroup at smc.vnet.net*Subject*: [mg19513] How to use sign[] for symbolic operations?*From*: LD <doellinger at t-online.de>*Date*: Sat, 28 Aug 1999 15:53:28 -0400*Sender*: owner-wri-mathgroup at wolfram.com

I use Mathematica 3.0. My question is how to solve the following problem: I define a < 0 and b < 0 (to intend that the sign is negative in both cases). Now I like to know the sign of the produkt of a*b, it should be positive by the hand. But how can I express it within mathematica 3.0? In general: how can I get the sign of an expression in symbolic form? In 1: Sign[a*b] ? Out 1: > 0 or positive. Many thanks for your help. Lothar Dillinger EMail: doellinger at t-online.de