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 A098541 #6 Jun 13 2017 22:13:23 %S A098541 1,4,20,140,1460,23884,639156,28895052,2260707508,311748794188, %T A098541 76846781851828,34240231941576524,27825184844063467700, %U A098541 41547951604100714698572,114705005843089453002549428 %N A098541 Column 2 of triangle A098539, which shifts left one column under the matrix square. %o A098541 (PARI) a(n)=local(A,B,C,m);A=matrix(1,1);A[1,1]=1; for(m=2,n+3,B=A^2;C=matrix(m,m);for(j=1,m, for(k=1,j, if(j<3 || k==j || k>m-1,C[j,k]=1,if(k==1,C[j,k]=B[j-1,1],C[j,k]=B[j-1,k-1])); ));A=C);A[n+3,3] %Y A098541 Cf. A098539, A098540. %K A098541 nonn %O A098541 0,2 %A A098541 _Paul D. Hanna_, Sep 13 2004