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Re: how to specify something is "real"

  • To: mathgroup at smc.vnet.net
  • Subject: [mg43166] Re: how to specify something is "real"
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sat, 16 Aug 2003 02:53:50 -0400 (EDT)
  • References: <bhi5j3$hne$1@smc.vnet.net>
  • Reply-to: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Sender: owner-wri-mathgroup at wolfram.com

"pgp" <bla at bla.com> wrote in message news:bhi5j3$hne$1 at smc.vnet.net...
> Hi,
> This is probably a dumb question, but I was trying to use Conjugate[] on
an
> expression that contains terms like Cos[x], and mathematica doesn't take
it
> for granted that Cos[x] is real, and hence the end result contain lots of
> Conjugate's that can't be reduced.
> Could someone tell me if it's possible to tell mathematica which parts of
> the expressions are supposed to be real, so it'll get rid of all those
> Conjugates?
> Thanks a lot.
> -K


On an expression by expression basis (ComplexExpand assumes that all
variables are real unless told otherwise):

In[1]:=
    ComplexExpand[Conjugate[Cos[x]]]
Out[1]=
    Cos[x]

Globally, We can use the package ReIm:

In[2]:=
    << Algebra`ReIm`
In[3]:=
    x /: Im[x] = 0; (*note the definition is for x, via x/:*)
In[4]:=
    Conjugate[Cos[x]]
Out[4]=
    Cos[x]

Allan
---------------
Allan Hayes
hay at haystack.demon.co.uk
Voice: +44 (0)116 241 8747
Fax: +44 (0)870 164 0565


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