       Re: assumption, supposition?

• To: mathgroup at smc.vnet.net
• Subject: [mg15500] Re: assumption, supposition?
• From: "Dale Horton" <daleh>
• Date: Mon, 18 Jan 1999 04:22:49 -0500
• Organization: Wolfram Research, Inc.
• References: <774kr1\$46h@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Another method is to use PowerExpand.

In:=
PowerExpand[Sqrt[x^2]]
Out=
x

-Dale

Erk Jensen wrote in message <774kr1\$46h at smc.vnet.net>...
>Probably I'm looking up the wrong keyword in Mathematica help, but I
>can't find what I'm looking for. Maybe that's because I've learned
>those expressions in german...
>
>My problem:
>
>I want Mathematica to assume that a certain condition is satisfied for
>the following algebraic transformations. More specifically: Let rho
>describe a radius coordinate. I know it is not negative real. So, for
>transformations of expressions containing rho, e.g.  Sqrt[rho^2], I
>want assure Mathematica that 0 <= rho is in fact satisfied, and that it
>is consequently allowed to replace Sqrt[rho^2] by rho. How do I do
>that?
>
>My solution was
>
>Unprotect[Sqrt]; Sqrt[rho_^2] := rho/; 0 <= rho; Protect[Sqrt];
>
>rho /: 0 <= rho = True;
>
>and this seems to work, but I wonder whether this is really the proper
>way. Since the TagSet I'm using here assigns the whole statement to
>rho, and ?rho results in
>
>Global`rho
>rho /: 0 <= rho = True
>
>so I still have my doubts ...
>
>Can some of you experts enlighten me?
>