Date: Dec 11, 2012 7:54 PM
Author: Christoph Lhotka
Subject: Re: TrigReduce: controlling the scope

Hello,

a workaround could be:

expr = Sin[alpha] Cos[omega tau1] Cos[omega tau2] Cos[beta]

step1 =
expr /. f_[arg_] :> Subscript[f, arg] /; MemberQ[{arg}, alpha | beta]

step2 = TrigReduce[step1]

step2 /. Subscript[f_, arg_] :> f[arg]


Best,

Christoph


On 12/11/2012 08:25 AM, alan wrote:
> I have an expression that is a sum of products of trignometric functions. Each term is something like this:
> Sin[alpha] Cos[omega tau1] Cos[omega tau2] Cos[beta]. (1)
> I want to apply trig identities to the terms that contain omega to transform them into trig functions of sums and differences, but I don't want the same transformation applied to the terms involving alpha and beta. For example, I want to express (1) as
> (1/2) Sin[alpha] Cos[beta](Cos[omega(tau1 - tau2)]+Cos[omega(tau1 + tau2)])
>
> If I apply TrigReduce to (1), I get terms like
> Cos[omega tau1 - omega tau2 + alpha - beta].
> How do I restrict the action of TrigReduce to terms containing omega?
> (I can do a hybrid calculation by cutting and pasting the terms I want, but I'd rather not have to cut and paste by hand).
>
> Thanks.
>