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 A181657 #7 Dec 26 2023 10:19:08 %S A181657 1,1,1,1,1,1,3,2,1,1,3,2,1,1,1,7,5,3,2,1,1,7,5,3,2,1,1,1,17,12,7,5,3, %T A181657 2,1,1,17,12,7,5,3,2,1,1,1,41,29,17,12,7,5,3,2,1,1,41,29,17,12,7,5,3, %U A181657 2,1,1,1 %N A181657 Generalized (conditional) Riordan array with k-th column generated by x^k * (1 + x - x^2 + x^3)/(1 - 2x^2 - x^4) if k mod 2 = 0, x^k * (1+x)/(1 - 2x^2 - x^4) otherwise. %C A181657 An eigentriangle of A181656. Even columns are essentially A001333 doubled. Odd columns are doubled Pell numbers (A000129). %C A181657 Row sums are A181658. %e A181657 Triangle begins %e A181657 1; %e A181657 1, 1; %e A181657 1, 1, 1; %e A181657 3, 2, 1, 1; %e A181657 3, 2, 1, 1, 1; %e A181657 7, 5, 3, 2, 1, 1; %e A181657 7, 5, 3, 2, 1, 1, 1; %e A181657 17, 12, 7, 5, 3, 2, 1, 1; %e A181657 17, 12, 7, 5, 3, 2, 1, 1, 1; %e A181657 41, 29, 17, 12, 7, 5, 3, 2, 1, 1; %e A181657 41, 29, 17, 12, 7, 5, 3, 2, 1, 1, 1; %K A181657 easy,nonn,tabl %O A181657 0,7 %A A181657 _Paul Barry_, Nov 03 2010