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MathGroup Archive 1999

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Why doesn't this work?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20352] Why doesn't this work?
  • From: Joe Strout <joe at strout.net>
  • Date: Sat, 16 Oct 1999 00:47:41 -0400
  • Organization: The Salk Institute
  • Sender: owner-wri-mathgroup at wolfram.com

I'm trying to understand why a temporary is needed in the following
case.  I've got a function of one parameter and two constants.  I
define the constants on the fly with "/.".  I want to find a root of
this equation.  When I use "/." in FindRoot, it doesn't work, but if I
create another function that has these as true constants (rather than
symbols), FindRoot works.  I don't understand why these two cases are
different.

Here's my function (a difference of gaussians):

Dog[t_] := (t/tau1)^6 * Exp[-6 * (t/tau1 - 1)] - (t/tau2)^6 * Exp[-6 *
(t/tau2 - 1)]

Here's my first attempt to find a root, and the result:

FindRoot[(Dog[t]==0) /. { tau1->13.7, tau2->24.8}, {t, 20}]
FindRoot::frnum: Function {False} is not a length 1 list of numbers at
{t} = {20.}.

Here, I define a temporary function, and a FindRoot which should come
out to the exact same thing as above -- but this one works:

Dog2[t_] = Dog[t] /. { tau1->13.7, tau2->24.8}
FindRoot[Dog2[t]==0, {t,20}]
{t->18.1648}

Why is this?  I've looked at it every way I can think of, thrown in
extra parens for grouping, checked that Dog2[t]==0 is exactly the same
as (Dog[t]==0) /. { tau1->13.7, tau2->24.8}, etc., but I can't figure
it out.  Any clues?

Thanks,
-- Joe

P.S. How do I copy a notebook in plain text format?  I tried "Copy As >
Plain Text" (and many others in that submenu) but it always gives me
text befuddled by lots of markup tags.  I had to paste the above line
by line, and fix the "->" and "==" symbols which came through as
foreign characters.

-- 
,------------------------------------------------------------------.
|    Joseph J. Strout           Biocomputing -- The Salk Institute |
|    joe at strout.net             http://www.strout.net              |
`------------------------------------------------------------------'
Check out the Mac Web Directory!    http://www.strout.net/macweb.cgi


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