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 A333809 #5 Apr 05 2020 23:28:18
%S A333809 0,1,1,1,1,0,1,0,1,0,1,1,1,0,2,0,1,1,1,-1,2,0,1,0,1,0,2,-1,1,2,1,-1,2,
%T A333809 0,2,0,1,0,2,0,1,0,1,-1,3,0,1,-1,1,1,2,-1,1,0,2,0,2,0,1,0,1,0,3,-1,2,
%U A333809 0,1,-1,2,2,1,-2,1,0,3,-1,2,0,1,-1,2,0,1,0,2,0,2,-2,1,2
%N A333809 G.f.: Sum_{k>=1} (-1)^(k + 1) * x^(k*(k + 1)) / (1 - x^k).
%C A333809 Number of odd divisors of n that are < sqrt(n) minus number of even divisors of n that are < sqrt(n).
%t A333809 nmax = 90; CoefficientList[Series[Sum[(-1)^(k + 1) x^(k (k + 1))/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
%Y A333809 Cf. A048272, A056924, A333781, A333805, A333810.
%K A333809 sign
%O A333809 1,15
%A A333809 _Ilya Gutkovskiy_, Apr 05 2020