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MathGroup Archive 2005

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Re: More strange behavior by ComplexExpand

  • To: mathgroup at smc.vnet.net
  • Subject: [mg60608] Re: [mg60603] More strange behavior by ComplexExpand
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 22 Sep 2005 02:08:04 -0400 (EDT)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

Although the expanded form is not the simplest form, it does not appear to 
be wrong. The different treatment of t occurs because t only appears as t^2 
and its Sign is immaterial. Whereas the undetermined Sign of a results in the 
(a^2)^(1/8) form and the unsimplified expression.

expr=(a/Pi)^(1/4) Exp[-(a t^2)/2];

ComplexExpand[expr]

((a^2)^(1/8)*Sqrt[E^(a*t^2)]*Cos[Arg[a]/4])/(E^(a*t^2)*Pi^(1/4)) + 
  (I*(a^2)^(1/8)*Sqrt[E^(a*t^2)]*Sin[Arg[a]/4])/(E^(a*t^2)*Pi^(1/4))

FullSimplify[%,Element[{a,t}, Reals]]

a^(1/4)/(E^((a*t^2)/2)*Pi^(1/4))

%==expr

True


Bob Hanlon

> 
> From: Raul Martinez <raulm231 at comcast.net>
To: mathgroup at smc.vnet.net
> Date: 2005/09/21 Wed AM 03:20:48 EDT
> Subject: [mg60608] [mg60603] More strange behavior by ComplexExpand
> 
> To Mathgroup,
> 
> I use Mathematica 5.2 with Mac OS X (Tiger).
> 
> Add the following to a recent thread on the sometimes strange  
> behavior of ComplexExpand.
> 
> I used ComplexExpand with an argument in which all the variables in  
> the argument of the function are real. Since ComplexExpand is  
> supposed to assume that all variables are real by default, one would  
> expect ComplexExpand to return the expression without change, but it  
> doesn't. Instead, here is what it does:
> 
> In[1]:=
> 
>      ComplexExpand[ (a / Pi)^(1/4) Exp[ (-(a t^2)/2 ] ]
> 
> Out[2]:=
> 
>      (Exp[-a t^2] Sqrt[Exp[a t^2]] (a^2)^(1/8) Cos[Arg[a] / 4]) / Pi^ 
> (1/4) + i (Exp[-a t^2] Sqrt[Exp[a t^2]] (a^2)^(1/8) Sin[Arg[a] /  
> 4]) / Pi^(1/4).
> 
> I have inserted parentheses in a few places to improve the legibility  
> of the expressions.
> 
> ComplexExpand treats the variable "a" as complex, but "t" as real.  
> This is puzzling to say the least. Moreover, it renders a^(1/4) as  
> (a^2)^(1/8), which seems bizarre.
> 
> My interest is not in obtaining the correct result, which is easy to  
> do. Rather, I bring this up as yet another example of the  
> unreliability of ComplexExpand. In case anyone is wondering why I  
> would use ComplexExpand on an expression I know to be real, the  
> reason is that the expression in question is a factor in a larger  
> expression that contains complex variables. Applied to the larger  
> expression, ComplexExpand returned an obviously incorrect expansion  
> that I traced to the treatment of the example shown above.
> 
> I welcome comments and suggestions.
> 
> Thanks in advance,
> 
> Raul Martinez
> 
> 
> 
> 
> 
> 
> 


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