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 A366049 #7 Sep 27 2023 10:07:35
%S A366049 1,2,8,39,211,1218,7349,45790,292361,1902834,12577737,84205212,
%T A366049 569788192,3890728052,26775751320,185525538183,1293171205833,
%U A366049 9061500578178,63794947215218,451028012126797,3200898741338041,22794860112273294,162841330273522907
%N A366049 Expansion of (1/x) * Series_Reversion( x*(1-x)^3/(1-x+x^2) ).
%F A366049 a(n) = (1/(n+1)) * Sum_{k=0..floor(n/2)} binomial(n+1,k) * binomial(3*n-k+1,n-2*k).
%o A366049 (PARI) a(n) = sum(k=0, n\2, binomial(n+1, k)*binomial(3*n-k+1, n-2*k))/(n+1);
%Y A366049 Cf. A005043, A109081, A366050.
%K A366049 nonn
%O A366049 0,2
%A A366049 _Seiichi Manyama_, Sep 27 2023