Re: force variable to be real

• To: mathgroup at smc.vnet.net
• Subject: [mg91778] Re: force variable to be real
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Sun, 7 Sep 2008 22:53:14 -0400 (EDT)
• References: <ga0744\$opl\$1@smc.vnet.net>

```phillman5 wrote:

> 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.

You could use *ComplexExpand[]*, which assumes that all the variables --
x and y in your example --  are real, in the definition of y.

In[1]:= w := x + I 2 x z
y := ComplexExpand[Re[w]]
D[y, x]

Out[3]= 1

Regards,
-- Jean-Marc

```

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