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 A139469 #21 Sep 08 2022 08:45:33
%S A139469 0,0,1,9,36,101,261,882,3921,17253,67554,243695,876789,3324906,
%T A139469 13166791,52301709,203824548,782913717,3010327497,11695756698,
%U A139469 45823049817,179787741723,703527078258,2747647985241,10739885115573,42082084255050,165225573240651
%N A139469 a(n) = Sum_{k=0..n} C(n,3k+2)^2.
%C A139469 The recurrence is same as for A119363. - _Vaclav Kotesovec_, Mar 12 2019
%H A139469 Seiichi Manyama, <a href="/A139469/b139469.txt">Table of n, a(n) for n = 0..1000</a>
%F A139469 a(n) ~ 4^n / (3*sqrt(Pi*n)). - _Vaclav Kotesovec_, Mar 12 2019
%t A139469 Table[Sum[Binomial[n, 3*k + 2]^2, {k, 0, n}], {n, 0, 40}] (* _Vaclav Kotesovec_, Mar 12 2019 *)
%o A139469 (PARI) a(n) = sum(k=0, n, binomial(n, 3*k+2)^2); \\ _Michel Marcus_, Mar 12 2019
%o A139469 (Magma) [&+[Binomial(n, 3*k+2)^2: k in [0..n]]: n in [0..30]]; // _Vincenzo Librandi_, Mar 14 2019
%Y A139469 Cf. A024495, A119363, A139468.
%K A139469 nonn
%O A139469 0,4
%A A139469 _N. J. A. Sloane_, Jun 12 2008