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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
>