This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A318431 #20 Aug 28 2018 19:26:44
%S A318431 1,3,6,30,126,603,3084,16614,91998,520779,3004206,17594250,104308092,
%T A318431 624801960,3775722348,22991162130,140928103134,868886416869,
%U A318431 5384796884934,33525472069566,209592226792326,1315211209647435,8281053081282900,52301607644921262
%N A318431 Number of n-element subsets of [3n] whose elements sum to a multiple of n.
%H A318431 Marko Riedel et al., <a href="https://math.stackexchange.com/questions/2894653/">Number of n-element subsets divisible by n</a>
%F A318431 a(n) = (-1)^n * (1/n) * Sum_{d|n} C(3d,d)*(-1)^d*phi(n/d) for n>0, a(0)=1.
%p A318431 with(numtheory); a := n -> `if`(n=0, 1, (-1)^n * 1/n * add(binomial(3*d,d)*(-1)^d*phi(n/d), d in divisors(n)));
%o A318431 (PARI) a(n) = if (n, (-1)^n * (1/n) * sumdiv(n, d, binomial(3*d,d)*(-1)^d*eulerphi(n/d)), 1); \\ _Michel Marcus_, Aug 27 2018
%Y A318431 Cf. A169888, A318432, A318433.
%Y A318431 Column k=3 of A304482.
%K A318431 nonn
%O A318431 0,2
%A A318431 _Marko Riedel_, Aug 26 2018