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 A331338 #4 Jan 14 2020 09:57:50
%S A331338 1,7,421,10375,1864921,177588115,43788506581,5364386887855,
%T A331338 3746015060385841,743097477083711275,562069712763196977901,
%U A331338 208700966175150043937095,201649817635812110915892361,89621739407126401163202051715,158792595367325562826629059282821
%N A331338 E.g.f.: Sum_{k>=1} (1 - sech(x^k)) (even powers only).
%F A331338 a(n) = -(2*n)! * Sum_{d|n} A028296(d) / (2*d)!.
%t A331338 nmax = 15; Table[(CoefficientList[Series[Sum[1 - Sech[x^k], {k, 1, nmax}], {x, 0, 2 nmax}], x] Range[0, 2 nmax]!)[[n]], {n, 1, 2 nmax + 1, 2}] // Rest
%t A331338 Table[-(2 n)! DivisorSum[n, EulerE[2 #]/(2 #)! &], {n, 1, 15}]
%Y A331338 Cf. A000364, A028296, A176474, A330255, A330528.
%K A331338 nonn
%O A331338 1,2
%A A331338 _Ilya Gutkovskiy_, Jan 14 2020