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 A181651 #16 Nov 11 2022 08:06:55 %S A181651 1,1,1,2,1,1,3,1,1,1,6,2,2,1,1,9,3,3,1,1,1,18,6,6,2,2,1,1,27,9,9,3,3, %T A181651 1,1,1,54,18,18,6,6,2,2,1,1,81,27,27,9,9,3,3,1,1,1,162,54,54,18,18,6, %U A181651 6,2,2,1,1 %N A181651 Eigentriangle of number triangle A070909. %C A181651 First column is (essentially) A038754. Row sums are A068911. Inverse is A181652. %C A181651 Generalized (conditional) Riordan array with k-th column generated by %C A181651 x^k*(1+x-x^2)/(1-3x^2) if k is even, %C A181651 (1+x-2x^2-x^3)/(1-3x^2) if k is odd. %H A181651 Paul Barry, <a href="http://arxiv.org/abs/1107.5490">Invariant number triangles, eigentriangles and Somos-4 sequences</a>, arXiv preprint arXiv:1107.5490 [math.CO], 2011. %e A181651 Triangle begins %e A181651 1; %e A181651 1, 1; %e A181651 2, 1, 1; %e A181651 3, 1, 1, 1; %e A181651 6, 2, 2, 1, 1; %e A181651 9, 3, 3, 1, 1, 1; %e A181651 18, 6, 6, 2, 2, 1, 1; %e A181651 27, 9, 9, 3, 3, 1, 1, 1; %e A181651 54, 18, 18, 6, 6, 2, 2, 1, 1; %e A181651 81, 27, 27, 9, 9, 3, 3, 1, 1, 1; %e A181651 162, 54, 54, 18, 18, 6, 6, 2, 2, 1, 1; %Y A181651 Cf. A038754, A068911, A070909, A181652. %K A181651 nonn,tabl %O A181651 0,4 %A A181651 _Paul Barry_, Nov 03 2010