# incomplete gamma function calculation

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Jamal Ahmad on 24 May 2013
Hi,
I have this equation:
gamma(5,x) = 2
how I can find the value of x?
Thanks
##### 2 CommentsShow NoneHide None
José-Luis on 24 May 2013
Matlab's gamma function only accepts one input argument.
Jamal Ahmad on 24 May 2013
this is incomplete gamma function, in matlab we can write as the following:
gammainc(a,x)
a = 5 and the result of the above function is 2 I need the value of x.

Sean de Wolski on 24 May 2013
Edited: Sean de Wolski on 24 May 2013
I don't believe that's possible:
gammainc(5,0)
gammainc(5,1000);
it never goes near two.
More
options = optimoptions(@fminunc,'tolfun',10^-10,'tolx',10^-12);
xv = fminunc(@(x)(gammainc(5,x)-(10^-4)).^2,4,options)
%xv =15.6926
gammainc(5,xv)
% ans =
% 1.0004e-04
Jamal Ahmad on 24 May 2013
Thank you very much. I use R2012a. Now it works.
options = optimset('tolfun',10^-10,'tolx',10^-12); xv = fminunc(@(x)(gammainc(5,x)-(10^-4)).^2,4,options) gammainc(5,xv)
Jamal Ahmad on 24 May 2013
now I have this warning:
Warning: Gradient must be provided for trust-region algorithm; using line-search algorithm instead. > In fminunc at 367 In test_2 at 12

### More Answers (2)

Walter Roberson on 24 May 2013
It appears to me that you will need to proceed numerically.
Jamal Ahmad on 24 May 2013
sure, but how

Matt J on 24 May 2013
xv = fzero(@(z) gammainc(5,z.^2)- 2 , sqrt(xguess)).^2;