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 A348951 #14 Aug 21 2025 12:32:01
%S A348951 0,1,-1,1,-1,0,-1,2,-1,0,-1,3,-1,0,-2,2,-1,1,-1,1,-2,0,-1,4,-1,0,-2,1,
%T A348951 -1,2,-1,3,-2,0,-2,2,-1,0,-2,4,-1,0,-1,1,-3,0,-1,5,-1,1,-2,1,-1,0,-2,
%U A348951 4,-2,0,-1,4,-1,0,-3,3,-2,0,-1,1,-2,2,-1,4,-1,0,-3,1,-2,0,-1,5
%N A348951 a(n) = Sum_{d|n, d < sqrt(n)} (-1)^(n/d).
%H A348951 Antti Karttunen, <a href="/A348951/b348951.txt">Table of n, a(n) for n = 1..20000</a>
%F A348951 G.f.: Sum_{k>=1} (-1)^(k + 1) * x^(k*(k + 1)) / (1 + x^k).
%F A348951 a(n) = A258998(n) - A348515(n). - _Ridouane Oudra_, Aug 21 2025
%t A348951 Table[DivisorSum[n, (-1)^(n/#) &, # < Sqrt[n] &], {n, 1, 80}]
%t A348951 nmax = 80; CoefficientList[Series[Sum[(-1)^(k + 1) x^(k (k + 1))/(1 + x^k), {k, 1, nmax}], {x, 0, nmax}], x] // Rest
%o A348951 (PARI) A348951(n) = sumdiv(n,d,if((d*d)<n,(-1)^(n/d),0)); \\ _Antti Karttunen_, Nov 05 2021
%Y A348951 Cf. A048272, A056924, A113652, A228441, A333809, A348515, A348952, A348953, A348954, A348955, A348956, A258998.
%K A348951 sign
%O A348951 1,8
%A A348951 _Ilya Gutkovskiy_, Nov 04 2021