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 A139468 #15 Mar 12 2019 10:29:50
%S A139468 0,1,4,9,17,50,261,1275,5028,17253,58601,218042,876789,3537847,
%T A139468 13783018,52301709,198627921,767778786,3010327497,11824753551,
%U A139468 46200429186,179787741723,700285942731,2738134757118,10739885115573,42164261091351,165467386466802
%N A139468 a(n) = Sum{k=0..n} C(n,3k+1)^2.
%C A139468 The recurrence is same as for A119363. - _Vaclav Kotesovec_, Mar 12 2019
%H A139468 Seiichi Manyama, <a href="/A139468/b139468.txt">Table of n, a(n) for n = 0..1000</a>
%F A139468 a(n) ~ 4^n / (3*sqrt(Pi*n)). - _Vaclav Kotesovec_, Mar 12 2019
%t A139468 Table[Sum[Binomial[n,3k+1]^2,{k,0,n}],{n,0,30}] (* _Harvey P. Dale_, Sep 08 2018 *)
%Y A139468 Cf. A024494, A119363, A139469.
%K A139468 nonn
%O A139468 0,3
%A A139468 _N. J. A. Sloane_, Jun 12 2008