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Re: PowerExpand in mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg31865] Re: [mg31843] PowerExpand in mathematica
• From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
• Date: Sat, 8 Dec 2001 05:51:40 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

Why is the correct answer -1/t (and why of course??).  Mathematica quite 
correctly gives:

In[24]:=
Simplify[(-1/t)^(3/2) Sqrt[-t],t>0]

Out[24]=
1
-
t

and

In[25]:=
Simplify[(-1/t)^(3/2) Sqrt[-t],t<0]

Out[25]=
   1
-(-)
   t

PowerExpand makes certain assumptions about the variables and in this 
cases the assumption was that t is positive. You made the equally 
arbitrary assumption that t is negative. Why is your arbitrary 
assumption "obviously correct"? (Or have you not heard about complex 
numbers?)

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Friday, December 7, 2001, at 07:56  PM, Peter wrote:

> Hi,
>
> Is this a known bug, feature? Or I am doing something wrong?
> -------------------------------
> Mathematica 4.0 for Linux
> Copyright 1988-1999 Wolfram Research, Inc.
>  -- Motif graphics initialized --
>
> In[1]:= PowerExpand[(-1/t)^(3/2) Sqrt[-t]]
>
>         1
> Out[1]= -
>         t
> ---------------------------------
> The correct answer is -1/t, of course.
>
> Thanks,
> Peter
>
>
>