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 A048099 #30 Feb 01 2020 18:37:11
%S A048099 0,0,0,3,15,45,105,315,1323,5355,18315,63855,272415,1264263,5409495,
%T A048099 22302735,101343375,507711375,2495918223,11798364735,58074029055,
%U A048099 309240315615,1670570920095,8792390355903,46886941456575,264381946998975,1533013006902975,8785301059346175,50439885753378303
%N A048099 Number of degree-n even permutations of order exactly 2.
%H A048099 Andrew Howroyd, <a href="/A048099/b048099.txt">Table of n, a(n) for n = 1..200</a>
%H A048099 Koda, Tatsuhiko; Sato, Masaki; Takegahara, Yugen; <a href="http://dx.doi.org/10.1142/S0219498815500528">2-adic properties for the numbers of involutions in the alternating groups</a>, J. Algebra Appl. 14 (2015), no. 4, 1550052 (21 pages).
%F A048099 a(n) = (A001189(n) + A051684(n))/2.
%F A048099 a(n) = Sum_{i=1..floor(n/4)} binomial(n,4i)(4i)!/(2^(2i)(2i)!). - _Luis Manuel Rivera MartÃnez_, May 16 2018
%F A048099 E.g.f.: (exp(x + x^2/2) + exp(x - x^2/2))/2 - exp(x). - _Andrew Howroyd_, Feb 01 2020
%t A048099 Table[Sum[Binomial[n , 4 i] (4 i)!/(2^(2 i) (2 i)!), {i, 1, Floor[n/4]}], {n,1,22}] (* _Luis Manuel Rivera MartÃnez_, May 16 2018 *)
%o A048099 (PARI) a(n) = sum(i=1, n\4, binomial(n,4*i)*(4*i)!/(2^(2*i)*(2*i)!)); \\ _Michel Marcus_, May 17 2018
%o A048099 (PARI) seq(n)={my(A=O(x*x^n)); Vec(serlaplace(exp(x + x^2/2 + A) + exp(x - x^2/2 + A) - 2*exp(x + A))/2, -n)} \\ _Andrew Howroyd_, Feb 01 2020
%Y A048099 Cf. A001189, A051695. A column of A057740.
%K A048099 easy,nonn
%O A048099 1,4
%A A048099 _Vladeta Jovovic_