This Wikipedia article states that the isomorphism type of a finite simple group is determined by its order, except that:
- L4(2) and L3(4) both have order 20160
- O2n+1(q) and S2n(q) have the same order for q odd, n > 2
I think this means that for each integer g, there are 0, 1 or 2 simple groups of order g.
Do we need the full strength of the Classification of Finite Simple Groups to prove this, or is there a simpler way of proving it?
(Originally asked at math.stackexchange.commath.stackexchange.com).