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 A337155 #11 Jan 28 2021 11:12:36
%S A337155 1,4,81,3644,291521,36440124,6559222321,1607009468644,514243029966081,
%T A337155 208268427136262804,104134213568131402001,63001199208719498210604,
%U A337155 45360863430278038711634881,38329929598584942711331474444,37563331006613243857104844955121,42258747382439899339242950574511124
%N A337155 a(n) = 5^n * (n!)^2 * Sum_{k=0..n} 1 / ((-5)^k * (k!)^2).
%F A337155 Sum_{n>=0} a(n) * x^n / (n!)^2 = BesselJ(0,2*sqrt(x)) / (1 - 5*x).
%F A337155 a(0) = 1; a(n) = 5 * n^2 * a(n-1) + (-1)^n.
%t A337155 Table[5^n n!^2 Sum[1/((-5)^k k!^2), {k, 0, n}], {n, 0, 15}]
%t A337155 nmax = 15; CoefficientList[Series[BesselJ[0, 2 Sqrt[x]]/(1 - 5 x), {x, 0, nmax}], x] Range[0, nmax]!^2
%o A337155 (PARI) a(n) = 5^n * (n!)^2 * sum(k=0, n, 1 / ((-5)^k * (k!)^2)); \\ _Michel Marcus_, Jan 28 2021
%Y A337155 Cf. A001908, A073701, A336808, A337152, A337153, A337154.
%K A337155 nonn
%O A337155 0,2
%A A337155 _Ilya Gutkovskiy_, Jan 27 2021