Re: L2 inner product. Integrate and Conjugate?

• To: mathgroup at smc.vnet.net
• Subject: [mg36330] Re: L2 inner product. Integrate and Conjugate?
• From: phbrf at t-online.de (Peter Breitfeld)
• Date: Mon, 2 Sep 2002 04:08:37 -0400 (EDT)
• References: <akpk8a\$s2s\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Andreas Dietrich schrieb:
> Hello.
>
> I am trying to implement the inner product in the space of
> complex-valued, square integrable functions over [-1/2,1/2], which can
> be expressed in Mathematica code as
>
> inner[f_Function,g_function]:=Integrate[Conjugate[f[x]]*g[x],{x,-1/2,1/2}]
>
> This is simple enough. Problem is, Mathematica seamingly cannot
> evaluate the Integral for even the simplest of functions:
> In[10]:=inner[#&,#&]
>
> Out[10]:=\!\(\[Integral]\_\(-\(1\/2\)\)\%\(1\/2\)\(x\ Conjugate[
>           x]\) \[DifferentialD]x\)
>
Andreas,
Conjugate does not evaluate the expression, if the variables aren't
known to be real. Your example (inner[#&,#&]) works if you define your
function using ComplexExpand:

inner[f_,g_]:=Integrate[ComplexExpand[Conjugate[f[x]]]g[x], ...]

You may also use a home-made Conjugate, I'll call it "Konjugiert", eg:

ruKonjugiert={Complex[re_,im_]:>Complex[re,-im]};
Konjugiert[ausdr__]:=ausdr /. ruKonjugiert;

Replacing Conjugate with Konjugiert in your Definition works for your
simple Example, but more komplex functions will need a ComplexExpand to
get the wanted real result.

Gruß Peter
--
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Peter Breitfeld, Bad Saulgau, Germany   Meinen GnuPG/PGP-5x Key gibts dort

```

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