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MathGroup Archive 2002

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Re: Simple Trigonometric Integrals

  • To: mathgroup at smc.vnet.net
  • Subject: [mg32366] Re: [mg32338] Simple Trigonometric Integrals
  • From: Tomas Garza <tgarza01 at prodigy.net.mx>
  • Date: Wed, 16 Jan 2002 03:30:37 -0500 (EST)
  • References: <200201150730.CAA04585@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

I guess, perhaps you might be able to help Mathematica a little if you allow
it to perform the integrations term by term, since you know that your
expression is a sum. The trouble is, I surmise, that the constant factors a,
b, c,... appear in two or more of the terms of the sum, and a lot of time is
consumed in trying to simplify the expression before the integration,
instead of proceeding to integrate term by term, which in this case is
immediate, due to the simple trigonometric nature of each of them. You see,

In[1]:=
Integrate[joe,{t,0,2 Pi}]//Timing
Out[1]=
{57.83 Second,0}

gives the right answer, but it takes a very long time, whereas

In[2]:=
Plus@@(Integrate[#,{t,0,2 Pi}]&/@List@@joe)//Timing
Out[2]=
{0.05 Second,0}

is reasonably fast.

Tomas Garza
Mexico City


----- Original Message -----
From: "Joe Helfand" <jhelfand at wam.umd.edu>
To: mathgroup at smc.vnet.net
Subject: [mg32366] [mg32338] Simple Trigonometric Integrals


> Hey,
>
>     I have a thing about Mathematica.  Sometimes I have a real long
> expression that involves the integral of the sum of lots of cosines and
> sines of some variable let's say 't'.  But having done some fancy maths
> on my own to reduce it and get into a simple integral from 0 to 2 Pi,
> and the sines and cosines all involve some integer multiple of t, the
> integration takes for ever, it basically hangs.  Now, although the
> expression is long, and there are a lot of terms in it, it still just
> becomes a simple periodic integral from zero to 2 pi, and all the
> trigonometric terms involving t should just drop out.  Kind of like what
> sometimes can happen if you are playing around with a Fourier series
> expansion (by the way, does Mathematica have a built in Fourier Series
> expansion?  I mean something like Series[], but returns fourier
> coefficients?).  Uptill now, I have been able to get by with something
> like using
>
> periodicIntegral={Cos[t] -> 0, Cos[2 t] -> 0, Cos[3 t] -> 0, Cos[4 t] ->
> 0, Cos[5t] -> 0, Cos[6 t] -> 0, Cos[7 t] -> 0, Sin[t] -> 0, Sin[2 t] ->
> 0, Sin[3 t] -> 0, Sin[4 t] -> 0, Sin[5t] -> 0, Sin[6 t] -> 0, Sin[7 t]
> -> 0};
>
> and then doing a replace on the expresion, multiplying the result by 2
> Pi.  But now I am in a bind where no amount of TrigReduce, TrigExpand,
> TrigFactor, etc. will get this big ass expression into the desired form
> where the above is approriate (because there are other sines and cosines
> of other variables that get put into the terms and stand by
> themselves).  Still, the expression should be easy to do for the
> computer, even I can go through and set these terms to zero, but it will
> just take me a long time.  An example of what I am talking about, just
> try the following:
>
> In[687]:=
> Joe = a c Cos[t]/(g s) + b q Cos[2 t]/(c f) + c Cos[3 t]/(d a) +
>       d f Cos[4 t]/(h a n) + e q Cos[5t]/(g a) + f l Cos[6 t]/(w r m) +
>       g b Cos[7 t]/(o n x) + h Sin[t]/(b c) + i Sin[2 t]/(h e r) +
>       j y Sin[3 t]/(l p) + d k Sin[4 t]/(j c) + l m a Sin[5 t]/(f s b h)
> +
>       m p Sin[6 t]/(k j) + q n Sin[7 t]/(x c);
>
> In[688]:=
> Integrate[Joe, {t, 0, 2 Pi}]
>
> and you willl see it takes a long time to integrate.  (It will
> eventually get done.)  I know this is just zero, but why does it take so
> long for the computer to figure out?  It is true that my expression is
> even longer than this one, so essentially it hangs, but basically it is
> the same problem.  I do not what to be hunting through my equation from
> hell setting all the relevant trigonometric terms to zero when the
> computer should be able to do this.  Well, sorry for the harangue but I
> greatly appreciate you reading down so far, really.  If you have any
> suggestions or comments, point out I am an idiot there is some simple
> thing in Mathematica, please send it.
>
> Joe
>
>



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