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 A177282 #17 Oct 27 2023 19:50:20 %S A177282 1,1,6,12,26,48,86,148,250,416,686,1124,1834,2984,4846,7860,12738, %T A177282 20632,33406,54076,87522,141640,229206,370892,600146,971088,1571286, %U A177282 2542428,4113770,6656256,10770086,17426404,28196554,45623024,73819646,119442740,193262458 %N A177282 Number of permutations of 2 copies of 1..n with all adjacent differences <= 1 in absolute value. %H A177282 Alois P. Heinz, <a href="/A177282/b177282.txt">Table of n, a(n) for n = 0..1000</a> (terms n=1..197 from R. H. Hardin) %F A177282 a(n) = (2n)!/2^n = A000680(n) for n<=2. %p A177282 a:= proc(n) option remember; `if`(n<4, [1$2, 6, 12][n+1], %p A177282 ((8*n-31)*a(n-1) -(4*n-19)*a(n-2) -(3*n-10)*a(n-3) %p A177282 +(2*n-10)*a(n-4)) / (3*n-11)) %p A177282 end: %p A177282 seq(a(n), n=0..40); # _Alois P. Heinz_, Jan 14 2016 %Y A177282 Column k=2 of A331562. %Y A177282 Cf. A000680. %K A177282 nonn,easy %O A177282 0,3 %A A177282 _R. H. Hardin_, May 06 2010