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 A325937 #12 Sep 20 2019 21:12:07
%S A325937 0,1,1,0,1,1,1,-1,2,1,1,-1,1,1,3,-2,1,1,1,-1,3,1,1,-3,2,1,3,-1,1,1,1,
%T A325937 -3,3,1,3,-2,1,1,3,-3,1,1,1,-1,5,1,1,-5,2,1,3,-1,1,1,3,-3,3,1,1,-3,1,
%U A325937 1,5,-4,3,1,1,-1,3,1,1,-5,1,1,5,-1,3,1,1,-5
%N A325937 Expansion of Sum_{k>=1} (-1)^(k + 1) * x^(2*k) / (1 - x^k).
%C A325937 Number of odd proper divisors of n minus number of even proper divisors of n.
%H A325937 Antti Karttunen, <a href="/A325937/b325937.txt">Table of n, a(n) for n = 1..65537</a>
%F A325937 G.f.: Sum_{k>=2} x^k / (1 + x^k).
%F A325937 a(n) = -Sum_{d|n, d<n} (-1)^d.
%F A325937 a(n) = A048272(n) + (-1)^n.
%t A325937 nmax = 80; CoefficientList[Series[Sum[(-1)^(k + 1) x^(2 k)/(1 - x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
%t A325937 Table[-DivisorSum[n, (-1)^# &, # < n &], {n, 1, 80}]
%o A325937 (PARI) A325937(n) = -sumdiv(n, d, if(d==n,0,((-1)^d))); \\ _Antti Karttunen_, Sep 20 2019
%Y A325937 Cf. A032741, A048272, A058344, A091954, A275495 (partial sums), A325939.
%K A325937 sign,look
%O A325937 1,9
%A A325937 _Ilya Gutkovskiy_, Sep 09 2019