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 A340887 #5 Jan 25 2021 19:04:54
%S A340887 1,1,7,99,2511,99531,5680125,441226521,44766049599,5748319130283,
%T A340887 911271895816077,174799606363478361,39903413238125862309,
%U A340887 10690643656077551475921,3321750648705212259711063,1184831658624977151885176859,480843465699932167142334581919
%N A340887 a(0) = 1; a(n) = Sum_{k=0..n-1} binomial(n,k)^2 * 3^(n-k-1) * a(k).
%F A340887 Sum_{n>=0} a(n) * x^n / (n!)^2 = 3 / (4 - BesselI(0,2*sqrt(3*x))).
%t A340887 a[0] = 1; a[n_] := a[n] = Sum[Binomial[n, k]^2 3^(n - k - 1) a[k], {k, 0, n - 1}]; Table[a[n], {n, 0, 16}]
%t A340887 nmax = 16; CoefficientList[Series[3/(4 - BesselI[0, 2 Sqrt[3 x]]), {x, 0, nmax}], x] Range[0, nmax]!^2
%Y A340887 Cf. A102221, A255927, A340886, A340888.
%K A340887 nonn
%O A340887 0,3
%A A340887 _Ilya Gutkovskiy_, Jan 25 2021