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MathGroup Archive 2004

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Help -- Weird integration behavior

  • To: mathgroup at
  • Subject: [mg49345] Help -- Weird integration behavior
  • From: miked378 at (Mike)
  • Date: Thu, 15 Jul 2004 07:00:08 -0400 (EDT)
  • Sender: owner-wri-mathgroup at

I'm working on a project that I last worked on a couple of years ago,
and I'm trying to pick up where I left off. I had been doing some
integrations using Mathematica 4.2, and all was well. I'm now trying
to use the same workbook, but with Mathematica 5.0, and the
integration that previously seemed to work now gives me a value of

Interestingly, when I run the same file on different versions of
Mathematica, I get either zero or an expression:

--v5.0 (student), Macintosh: zero
--v5.0, Solaris: zero
--v4.2, Macintosh: expression
--v4.0, Windows: expression

Here's the code:
F[x_, t_] = (ks (UnitStep[x] - UnitStep[x - 0.5]))(UnitStep[t] -
UnitStep[t - 0.5]);
phi[n_, x_] = Sqrt[2] Sin[n Pi x/L]
G[n_, t_] = Integrate[F[x, t] phi[n, x], {x, 0, L}]

It's the output of this last line that's interesting to me -- either
an expression (I hope) or zero. My questions are:

-- Did something change in 5.0 that makes it so that this integration
doesn't work correctly?
-- Can anybody out there reproduce my problem (i.e., run these three
cells on different versions and produce different answers?)

Thanks for the help!

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