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 A249391 #16 Feb 23 2024 11:01:26 %S A249391 1,1,0,0,0,0,0,0,1,16,272,2880,32648,333696,3617545,38942336, %T A249391 443291226,5168682752,63523977424,807890646528,10790865576074, %U A249391 149552304296960,2168856599259491,32631738808727552,512002155247238968 %N A249391 The number of down-up permutations of [n] where adjacent elements differ by at least 4. %Y A249391 Cf. A245377 (at least 2), A249390 (at least 3), A249392-A249393, A179957 (non-alternating). %K A249391 nonn,more %O A249391 0,10 %A A249391 _R. J. Mathar_, Oct 27 2014 %E A249391 a(17)-a(20) from _Alois P. Heinz_, Oct 27 2014 %E A249391 a(21)-a(24) from _Max Alekseyev_, Feb 18 2024