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 A320392 #22 Feb 16 2025 08:33:56 %S A320392 1,1,0,0,4,4,10,24,252,410,1998,7798,65188,280582,2281108,10585748, %T A320392 110903088 %N A320392 Number of permutations of 3 indistinguishable copies of 1,...,n such that there are exactly j numbers between the first and the third copy of j and floor(j/2) numbers between the first and the second or between the second and the third copy of j. %C A320392 a(n) is even for n > 1. %H A320392 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LangfordsProblem.html">Langford's Problem</a> %H A320392 Wikipedia, <a href="https://en.wikipedia.org/wiki/Dancing_Links">Dancing Links</a> %H A320392 Wikipedia, <a href="https://en.wikipedia.org/wiki/Langford_pairing">Langford pairing</a> %e A320392 a(1) = 1: 111. %e A320392 a(4) = 4: 111224234343, 111343432422, 224234343111, 343432422111. %Y A320392 Cf. A285527, A322153. %K A320392 nonn,more %O A320392 0,5 %A A320392 _Alois P. Heinz_, Dec 06 2018