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 A027209 #15 Oct 26 2019 11:10:48
%S A027209 1,7,34,173,741,3549,14670,68683,279715,1294598,5233060,24052965,
%T A027209 96822587,442972139,1778697948,8110611295,32516501991,147897243768,
%U A027209 592346459071,2688895275974,10762289869782,48776681304283,195143578562778,883272843800993,3532739195660162
%N A027209 a(n) = Sum_{k=0..n-2} T(n,k) * T(n,k+2), with T given by A026725.
%p A027209 A027209 := proc(n)
%p A027209 add( A026725(n,k)*A026725(n,k+2),k=0..n-2) ;
%p A027209 end proc: # _R. J. Mathar_, Oct 21 2019
%K A027209 nonn
%O A027209 2,2
%A A027209 _Clark Kimberling_
%E A027209 Name corrected by _R. J. Mathar_, Oct 21 2019
%E A027209 More terms from _Sean A. Irvine_, Oct 24 2019