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 A373719 #8 Jun 15 2024 09:23:06
%S A373719 1,1,1,4,9,26,91,281,1105,4105,16576,70643,301405,1382928,6363876,
%T A373719 30605836,150820769,758835104,3941917840,20787546715,112615930451,
%U A373719 620969188400,3492709446326,20034747631656,116780977502105,693539635192626,4181549476945504,25627647913369903
%N A373719 a(n) = Sum_{k=0..floor(n^2/(2*n+1))} binomial(n * (n-2*k),k).
%F A373719 a(n) = [x^n] 1/(1 - x * (1 + x^2)^n).
%o A373719 (PARI) a(n) = sum(k=0, n^2\(2*n+1), binomial(n*(n-2*k), k));
%Y A373719 Main diagonal of A373717.
%Y A373719 Cf. A099237.
%K A373719 nonn
%O A373719 0,4
%A A373719 _Seiichi Manyama_, Jun 15 2024