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 A333781 #9 Apr 06 2020 00:17:51
%S A333781 1,1,1,0,1,0,1,0,2,0,1,1,1,0,2,-1,1,1,1,-1,2,0,1,0,2,0,2,-1,1,2,1,-1,
%T A333781 2,0,2,-1,1,0,2,0,1,0,1,-1,3,0,1,-1,2,1,2,-1,1,0,2,0,2,0,1,0,1,0,3,-2,
%U A333781 2,0,1,-1,2,2,1,-2,1,0,3,-1,2,0,1,-1,3,0,1,0,2,0,2,-2,1,2
%N A333781 G.f.: Sum_{k>=1} (-1)^(k + 1) * x^(k^2) / (1 - x^k).
%C A333781 Number of odd divisors of n that are <= sqrt(n) minus number of even divisors of n that are <= sqrt(n).
%t A333781 nmax = 90; CoefficientList[Series[Sum[(-1)^(k + 1) x^(k^2)/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
%o A333781 (PARI) a(n) = sumdiv(n, d, if (d^2<=n, if (d%2, 1, -1))); \\ _Michel Marcus_, Apr 05 2020
%Y A333781 Cf. A038548, A048272, A069288, A333782.
%K A333781 sign
%O A333781 1,9
%A A333781 _Ilya Gutkovskiy_, Apr 05 2020