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 A308647 #6 Feb 16 2025 08:33:55
%S A308647 -1,1,-2,-9,267,-2180,-50533,1966797,8638718,-2540956509,27172288399,
%T A308647 5592543175252,-168392610536153,-20819319685262839,
%U A308647 1122009166836993406,127595724180314195839,-9985347479130060737373,-1244077225312583088164916,120225865637787689310572899,18462990063073814590636032245
%N A308647 a(n) = exp(1) * Sum_{k>=0} (-1)^k*k^(2*n+1)/k!.
%H A308647 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/ComplementaryBellNumber.html">Complementary Bell Number</a>
%F A308647 a(n) = Sum_{k=0..2*n+1} (-1)^k*Stirling2(2*n+1,k).
%F A308647 a(n) = A000587(2*n+1).
%t A308647 Table[Exp[1] Sum[(-1)^k k^(2 n + 1)/k!, {k, 0, Infinity}], {n, 0, 19}]
%t A308647 Table[BellB[2 n + 1, -1], {n, 0, 19}]
%Y A308647 Cf. A000587, A099977, A308646.
%K A308647 sign
%O A308647 0,3
%A A308647 _Ilya Gutkovskiy_, Jun 13 2019