Re: How to force Mathematica to treat a number as positive and real?

*To*: mathgroup at smc.vnet.net*Subject*: [mg51895] Re: How to force Mathematica to treat a number as positive and real?*From*: "Steve Luttrell" <steve_usenet at _removemefirst_luttrell.org.uk>*Date*: Thu, 4 Nov 2004 01:50:47 -0500 (EST)*References*: <cm9vi3$8nc$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

This works: Simplify[Abs[a],a>0] and this does too: Assuming[a>0,Simplify[Abs[a]]] Have a look at the Help Browser for more information on Assuming. Steve Luttrell "Rainer" <wilhelm.rainer at gmx.net> wrote in message news:cm9vi3$8nc$1 at smc.vnet.net... > I'd like to do some symbolic computation with parameters. These > parameters (named, e.g. "a") are always *real-valued* and positive. I > managed to tell Mathematica that a is a real number by using: > > a/:Im[a]=0; > > This works since the response to "Re[a]" is now "a" (and not "Re[a]" > which is the general result when one does not use the above definition > of a). > > Now, I would like to do a similar thing, forcing Mathematica to assume > that a is a positive number. I want that the result of Abs[a] is equal > to a. I tried: > > a/:Positive[a]=True; > > but it does not work. Does anybody know how I can do this? > Thanks > Rainer >