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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