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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 |
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