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 A321956 #11 Feb 16 2025 08:33:57 %S A321956 1,1,0,2,4,0,16,40,0,456,1759,0,34636,175198,0,5494621,34043062,0 %N A321956 Number of permutations of 3 indistinguishable copies of 1,...,n such that the first and second copies of j are adjacent and there are exactly j-1 numbers between the second and the third copy of j. %H A321956 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LangfordsProblem.html">Langford's Problem</a> %H A321956 Wikipedia, <a href="https://en.wikipedia.org/wiki/Dancing_Links">Dancing Links</a> %H A321956 Wikipedia, <a href="https://en.wikipedia.org/wiki/Langford_pairing">Langford pairing</a> %F A321956 a(n) = 0 for n == 2 (mod 3). %e A321956 a(3) = 2: 111332232, 332232111. %e A321956 a(4) = 4: 111334432242, 332232441114, 334432242111, 441114332232. %Y A321956 Cf. A264813, A322153. %K A321956 nonn,more %O A321956 0,4 %A A321956 _Alois P. Heinz_, Nov 22 2018