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 A187121 #6 Mar 30 2012 18:37:26
%S A187121 1,6,42,377,4248,58269,947117,17848872,383237040,9243654925,
%T A187121 247586590398,7293962774574,234458181733224,8167132024738422,
%U A187121 306500617604837898,12329458457556027538,529269910501209999900
%N A187121 A diagonal of triangle A187120.
%C A187121 Definition of triangle: A187120(n,k) = [y^k] R_{n-1}(y+y^2) for k=3..n, n>3, where R_n(y) is the n-th row polynomial starting with R_3(y)=y^3.
%F A187121 Equals column 2 in the matrix square of triangle A135080.
%o A187121 (PARI) {a(n)=local(Rn=y^3);for(m=3,n,Rn=subst(truncate(Rn),y,y+y^2+O(y^m)));polcoeff(Rn,n,y)}
%Y A187121 Cf. A187120, A135083, A187122, A187123, A187124.
%K A187121 nonn
%O A187121 3,2
%A A187121 _Paul D. Hanna_, Mar 08 2011