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 A322153 #10 Feb 16 2025 08:33:57 %S A322153 1,1,0,0,0,0,5,12,0,0,0,0,2967,13275,0,0,0,0,38182584,222784153,0,0,0, %T A322153 0 %N A322153 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 second and the third copy of j. %H A322153 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/LangfordsProblem.html">Langford's Problem</a> %H A322153 Wikipedia, <a href="https://en.wikipedia.org/wiki/Dancing_Links">Dancing Links</a> %H A322153 Wikipedia, <a href="https://en.wikipedia.org/wiki/Langford_pairing">Langford pairing</a> %F A322153 a(6) = 5: 111223236356454654, 111224245645635363, 223235345465461116, 223236356454654111, 224245645635363111. %Y A322153 Cf. A321956, A320392. %K A322153 nonn,more %O A322153 0,7 %A A322153 _Alois P. Heinz_, Nov 28 2018