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 A012821 #11 Feb 25 2018 20:36:54
%S A012821 1,1,1,3,9,45,225,1071,6993,56697,467745,3279419,29529753,376156261,
%T A012821 4297350785,35274762407,378759492257,8003761282417,122599030798017,
%U A012821 853554115621363,8002975063348265,450081179390735645
%N A012821 Expansion of e.g.f. exp(sec(x)*arcsinh(x)).
%C A012821 The first negative number is a(24) = -686072407102337430543.
%H A012821 Andrew Howroyd, <a href="/A012821/b012821.txt">Table of n, a(n) for n = 0..200</a>
%e A012821 exp(sec(x)*arcsinh(x)) = 1 + x + 1/2!*x^2 + 3/3!*x^3 + 9/4!*x^4 + 45/5!*x^5 + ...
%o A012821 (PARI) seq(n)={my(x='x+O('x^n)); Vec(serlaplace(exp(asinh(x)/cos(x))))} \\ _Andrew Howroyd_, Feb 25 2018
%K A012821 sign
%O A012821 0,4
%A A012821 Patrick Demichel (patrick.demichel(AT)hp.com)