There is a lot of statistical criteria/tests (statistics) named after a statistician/mathematician/biologist/economist, etc. But for instance, this and this examples have different spelling...
Do I have to write an apostrophe after a surname in such a situation or not?
There's an interesting discussion on this topic in Wikipedia talk.
Go with tradition if you can find it, or with whatever manual of style you follow otherwise.
It depends on how you refer to it, eg. "Fermat's last theorem" but "the Goldbach conjecture"