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 A249390 #17 Feb 24 2024 00:49:10 %S A249390 1,1,0,0,0,0,1,8,68,480,3637,27968,231410,2014784,18789010,185234176, %T A249390 1943243785,21501237248,251589065328,3093350098432,40007001803593, %U A249390 541652523341824,7679190979467688,113578314941202432 %N A249390 The number of down-up permutations of [n] where adjacent elements differ by at least 3. %Y A249390 Cf. A245377 (at least 2), A249391, A249392, A249393, A127697 (non-alternating). %K A249390 nonn,more %O A249390 0,8 %A A249390 _R. J. Mathar_, Oct 27 2014 %E A249390 a(17)-a(20) from _Alois P. Heinz_, Oct 27 2014 %E A249390 a(21) from _Alois P. Heinz_, Oct 28 2014 %E A249390 a(22)-a(23) from _Max Alekseyev_, Feb 20 2024