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 A343843 #13 May 06 2021 12:21:34
%S A343843 1,-1,1,-9,33,-241,1761,-15929,161473,-1853281,23584321,-330371049,
%T A343843 5047404513,-83546832721,1489242229281,-28442492633369,
%U A343843 579425286625153,-12541705195066561,287434687338368641,-6953491183101074889,177069197398959999393,-4734481603905334522801
%N A343843 a(n) = Sum_{k=0..n} (-1)^k*binomial(n, k)*A000831(k).
%F A343843 a(n) = (-2)^n*Sum_{k=0..n} A109449(n, k)*(-1/2)^k.
%F A343843 From _Vaclav Kotesovec_, May 06 2021: (Start)
%F A343843 a(n) ~ (-1)^n * exp(-Pi/4) * 4^(n+1) * n! / Pi^(n+1).
%F A343843 E.g.f.: exp(x)*(1 - tan(x))/(1 + tan(x)). (End)
%p A343843 a := n -> add((-1)^k*binomial(n, k)*A000831(k), k=0..n):
%p A343843 seq(a(n), n = 0..21);
%t A343843 Table[-1 + Sum[(-1)^k * Binomial[n, k] * 4^k * Abs[EulerE[k,1/2] + EulerE[k,1]], {k,0,n}], {n,0,20}] (* _Vaclav Kotesovec_, May 06 2021 *)
%Y A343843 Cf. A000831, A000834, A109449.
%K A343843 sign
%O A343843 0,4
%A A343843 _Peter Luschny_, May 06 2021