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Re: force variable to be real

  • To: mathgroup at smc.vnet.net
  • Subject: [mg91787] Re: force variable to be real
  • From: "David Park" <djmpark at comcast.net>
  • Date: Sun, 7 Sep 2008 22:54:52 -0400 (EDT)
  • References: <ga0744$opl$1@smc.vnet.net>

When working with complex expressions one always wants to keep ComplexExpand 
handy. You will almost always have to use it.

Re[x + I 2 x z] // ComplexExpand
D[%, x]
x
1

-- 
David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/


"phillman5" <PHILLMAN5 at gmail.com> wrote in message 
news:ga0744$opl$1 at smc.vnet.net...
> How do you force variables to be real?
>
> I have
>                w:=x + I 2 x z,
> and
>                y:=Re[w]
>
> and want the derivative of y wrt x,    x and z are Real
>                D[y, x]
> gives
>                Re'[x] - 2 z Im'[x z]
>
> where I'd expect  just 1.  Or I need to evaluate Re[w] forcing x and z
> to be real, then take the derivative.
>
> Thank you for any help
> Paul
>
> 



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