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From: Jason Bonior on 17 Jun 2010 12:32
Does anyone know if there is a function in Matlab which can take a
function and determine what the value of the input variable was based
on the output of the function?

A simple example:

y = 4*t^2

detemine t given y = 36

All I need is a numeric value, I am not interested in a symbolic
solution. I am working with a funciton of multiple gamma functions so
it is not easy for me to rearrange the equation to use interp1.

Thanks
From: Matt Fig on 17 Jun 2010 12:39
y = @(t) 4*t.^2
fzero(@(t) y(t)-36,5)
From: the cyclist on 17 Jun 2010 12:55
"Matt Fig" <spamanon(a)yahoo.com> wrote in message <hvdj39$pnl$1(a)fred.mathworks.com>...
> y = @(t) 4*t.^2
> fzero(@(t) y(t)-36,5)

Jason,

Matt's reply is spot-on correct, but I can see how you might be perplexed by it, if you have never used certain elements of MATLAB. If you have never heard of "function handles" (using "@"), you should look that up in the documentation. You might want read "doc fzero" as well, for the syntax of that command. (That will explain why the rather out-of-place value of 5 is in there, and how Matt's answer found the positive root rather than the negative root of your equation.)

the cyclist
From: Jason Bonior on 17 Jun 2010 12:56
On Jun 17, 12:39 pm, "Matt Fig" <spama...(a)yahoo.com> wrote:
> y = @(t) 4*t.^2
> fzero(@(t) y(t)-36,5)

I apologize but it seems I was not clear. Forget my simple example, my
actual problem is:

P = gammainc(N/2, x) / gamma(N/2)

N is an integer and x is my variable. I am looking for a way to find
the corresponding value of x given a desired value of P. For example
what value of x will give me P=0.9?
From: Jason Bonior on 17 Jun 2010 13:03
Looks like I replied too soon. I thought Matt was only answering my
simple problem lol. I have been reading the doc fzero and now
understand what you are getting at.

Thanks a lot guys!
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