Subject: double integrals seem to give different answers
From: Richard Hennessy
Date: Sat, 21 Nov 2009 17:52:34 -0500
This is a bug, there is no way calling trigsimp() should change the answer,
although it might express the answer in a simpler form. You should file a
bug report.
Rich
--------------------------------------------------
From: "G. Jay Kerns" <gkerns at ysu.edu>
Sent: Friday, November 20, 2009 9:36 PM
To: "maxima" <maxima at math.utexas.edu>
Subject: double integrals seem to give different answers
> Dear Maxima list,
>
> Let
>
> f(x,y,z) := exp((x^2 + y^2 +z^2)^(3/2));
>
> x: rho * sin(phi) * cos(theta);
> y: rho * sin(phi) * sin(theta);
> z: rho * cos(phi);
> J: rho^2 * sin(phi);
>
> Then the following double integrals give different answers... why?
>
> integrate(integrate(f(x,y,z) * J, rho,0,1), theta,0,2*%pi);
>
> integrate(integrate(trigsimp(f(x,y,z)) * J, rho,0,1), theta,0,2*%pi);
>
> (The second one is the correct answer). By the way, the inner
> integrals give the same answer although the first one without trigsimp
> is long and complicated.
>
> It would seem the moral of the story is, "Do as much simplification as
> possible before calling integrate." Is this true, or is there a more
> general principle that I am missing?
>
> I would appreciate any insight.
>
> Cheers,
> Jay
>
>
>
> ***************************************************
> G. Jay Kerns, Ph.D.
> Associate Professor
> Department of Mathematics & Statistics
> Youngstown State University
> Youngstown, OH 44555-0002 USA
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