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how to pull real numbers out of a linear funktion ?

• To: mathgroup@smc.vnet.net
• Subject: [mg11092] how to pull real numbers out of a linear funktion ?
• From: Roland Bollmann <roland@arnold.fkp.physik.th-darmstadt.de>
• Date: Sun, 22 Feb 1998 14:55:17 -0500

I want to define a function f[x_] with the property, that

f[c_ x_] := c f[x] whenever c is a real number

and

f[c_ x_] := -c f[x] whenever c is a komplex number.

It semms to me, that mathematica can not give a variable the   attribute
"real number" or "complex number".

But it should be easy ... somehow ;-)

Roland Bollmann (student of physics)