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 A309531 #21 Aug 06 2019 17:40:07
%S A309531 1,1,0,0,2,2,-3,1,5,5,-2,-2,4,10,-6,-6,1,1,-8,0,10,10,-16,-8,4,18,7,7,
%T A309531 13,13,-19,-7,9,19,-11,-11,7,21,-15,-15,-5,-5,-20,10,32,32,-36,-24,-1,
%U A309531 17,0,0,18,32,-14,6,34,34,-23,-23,7,45,-29,-13,5,5,-16,8,30,30,-66
%N A309531 a(n) = Sum_{k=2..n} (-1)^k * A240236(n, k).
%H A309531 Seiichi Manyama, <a href="/A309531/b309531.txt">Table of n, a(n) for n = 2..1000</a>
%F A309531 a(n) = a(n-1) if and only if n is an odd prime.
%e A309531 a(2) = 1.
%e A309531 a(3) = 2 - 1 = 1.
%e A309531 a(4) = 1 - 2 + 1 = 0.
%e A309531 a(5) = 2 - 3 + 2 - 1 = 0.
%e A309531 a(6) = 2 - 2 + 3 - 2 + 1 = 2.
%o A309531 (PARI) {a(n) = sum(k=2, n, (-1)^k*sumdigits(n, k))}
%Y A309531 Cf. A000040, A043306, A065091, A240236.
%K A309531 sign,base
%O A309531 2,5
%A A309531 _Seiichi Manyama_, Aug 06 2019