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Re: Factoring

• To: mathgroup at smc.vnet.net
• Subject: [mg65107] Re: Factoring
• From: "J Siehler" <jsiehler at gmail.com>
• Date: Wed, 15 Mar 2006 06:28:18 -0500 (EST)
• References: <dv6847$nq4$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Matt wrote:
> Hello Mathgroup,
>   I'm sure that I've overlooked something obvious, but for the past two
> and a half hours, I've been trying to figure out how to use built-in
> Mathematica functions to just factor the imaginary number 'I' out of
> this:
>
> 4*I*r^2*Sin[2*θ] + I*r^5*Sin[5*θ]

This will do the job for your specific case:

x = 4*I*r^2*Sin[2*θ] + I*r^5*Sin[5*θ]
Simplify[x, Count[#, _Complex, Infinity] &]

Pulling out a common subexpression in general seems tricky since the
desired subexpression may occur in different guises in different
summands.  For example, look at the FullForm of 'x' in the preceding;
there isn't really an 'I' in the first summand.