#### chi-square distribution [Eqn]

The Χ^{2} distribution plays an incredibly important role in astronomical data analysis, but it is pretty much a black box to most astronomers. How many people know, for instance, that its form is exactly the same as the γ distribution? A Χ^{2} distribution with ν degrees of freedom is

p(z|ν) = (1/Γ(ν/2)) (1/2), where^{ν/2}z^{ν/2-1}e^{-z/2}≡ γ(z;ν/2,1/2)z=Χ^{2}.