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 A038020 #19 Feb 02 2023 15:26:12 %S A038020 0,0,0,2,0,2,9,54,0,30,15376,12664,14304,4096,3187 %N A038020 Triangle: T(n,k), k<=n: groupoids with a nontrivial symmetry with n elements and k idempotents. %H A038020 <a href="/index/Gre#groupoids">Index entries for sequences related to groupoids</a> %F A038020 Difference of A038018 and A038019. %e A038020 Triangle begins: %e A038020 0; %e A038020 0, 0; %e A038020 2, 0, 2; %e A038020 9, 54, 0, 30; %e A038020 15376, 12664, 14304, 4096, 3187; %e A038020 ... %Y A038020 Row sums are A030246. %Y A038020 Main diagonal is A030249. %Y A038020 Columns k=0..1 are A030252, A030254. %Y A038020 Cf. A001329, A030248, A030251, A030271. %K A038020 nonn,tabl,more %O A038020 0,4 %A A038020 _Christian G. Bower_, May 15 1998