Note: Whilst MATL was only released on December 12, after this challenge was posted, I came up with this answer on December 9 using a pre-release MATL compiler, which is almost identical to the released compiler, this code is the same. — DavidDec 13 '15 at 23:54
In math.SE as well as in codegolf.SE we quite often use OEIS (Online Encyclopedia of Integer Sequences), which the standard reference for sequences in combinatorics and number theory.
I think - and apparently I am not the only one - this would be an excellent addition of sites that do get onebox...