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 A334125 #10 May 12 2020 11:55:15
%S A334125 2,2,2,2,4,6,10,18,30,54,98,178,328,608,1130,2114,3974,7490,14170,
%T A334125 26890,51150,97542,186420,356962,684784,1315870,2532410,4880646,
%U A334125 9418806,18199014,35204650,68174116,132152842,256415958,497967282,967879954,1882725390,3665038872
%N A334125 Number of subsets of {1, 3, ..., 2*n-1} which sum to 0 modulo 2*n-1.
%H A334125 Robert Israel, <a href="/A334125/b334125.txt">Table of n, a(n) for n = 1..1000</a>
%F A334125 If 2*k - 1 is a prime, then a(k) = (2^k - 2*(-1)^floor(k/2))/(2*k - 1).
%F A334125 Conjecture: a(n) = 2*abs(A178738(n)).
%e A334125 a(5) = 4 because there are 4 subsets of {1, 3, 5, 7, 9} which sum to 0 modulo 9: {}, {9}, {1, 3, 5}, {1, 3, 5, 9}.
%p A334125 f:= proc(n) local V, k;
%p A334125 V:= Vector(2*n-1);
%p A334125 V[2*n-1]:= 1;
%p A334125 for k from 1 to 2*n-1 by 2 do
%p A334125 V:= V + V[[$(k+1)..(2*n-1),$1..k]]
%p A334125 od;
%p A334125 V[2*n-1]
%p A334125 end proc:
%p A334125 map(f, [$1..40]); # _Robert Israel_, May 12 2020
%o A334125 (PARI) a(n) = {my(v=Vec(prod(i=1, n, x^(2*i-1)+1))); sum(i=0, n^2\(2*n-1), v[n^2+1-i*(2*n-1)]); }
%Y A334125 Cf. A063776, A178738.
%K A334125 nonn
%O A334125 1,1
%A A334125 _Jinyuan Wang_, Apr 30 2020