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# Re: how to pull real numbers out of a linear funktion ?
Clear[f];
f[c_ x_] /; (Head[c] === Real || Head[c] === Integer) := c f[x]
f[c_ x_] /; (Head[c] === Complex) := -c f[x]
{f[2x],f[2.x],f[(3+4I)x],f[8], f[2(3+4I)],f[(6+7I)(3+4I)],f[x y]}
{2 f[x],2. f[x],(-3-4 I) f[x],f[8],f[6+8 I],f[-10+45 I],f[x y]}
It is not clear what you want done if the factors are implied or the
order of the factors is ambiguous. For example,
Is f[8] supposed to be f[8] or 8 * f[1] or 4 f[2] or 2 f[4]?
Is f[2(3+4I)] supposed to be f[6+8I] or 2 * f[3+4I] or (-3-4I) * f[2]
or 2
* (-3-4I) * f[1] ?
Is x supposed to be restricted to being a symbol? If so,
Clear[f];
f[c_ x_Symbol] /; (Head[c] === Real || Head[c] === Integer) := c f[x]
f[c_ x_Symbol] /; (Head[c] === Complex) := -c f[x]
Bob Hanlon
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