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 A371873 #8 Apr 10 2024 08:46:58
%S A371873 1,3,10,36,135,517,2003,7815,30634,120480,475002,1876294,7422676,
%T A371873 29400192,116567356,462561572,1836843591,7298613997,29016050831,
%U A371873 115408159467,459209330821,1827849895817,7277945888781,28986847296997,115479393316211,460159673245743
%N A371873 a(n) = Sum_{k=0..floor(n/3)} binomial(2*n+1,n-3*k).
%F A371873 a(n) = [x^n] 1/(((1-x)^3-x^3) * (1-x)^(n-1)).
%o A371873 (PARI) a(n) = sum(k=0, n\3, binomial(2*n+1, n-3*k));
%Y A371873 Cf. A360150, A371871, A371872.
%Y A371873 Cf. A002450, A360168, A371777.
%K A371873 nonn
%O A371873 0,2
%A A371873 _Seiichi Manyama_, Apr 10 2024