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 A177284 #13 May 30 2025 16:57:43 %S A177284 1,1,6,90,2520,41580,516180,6068622,76331906,958679970,11679900408, %T A177284 138047313960,1610654864328,18649754961744,214256589488616, %U A177284 2439692058769566,27562317214408488,309367132582535226,3453299423388485028,38354816922190327314,424048220090513056908 %N A177284 Number of permutations of 2 copies of 1..n with all adjacent differences <= 3 in absolute value. %H A177284 Andrew Howroyd, <a href="/A177284/b177284.txt">Table of n, a(n) for n = 0..200</a> %F A177284 a(n) = (2n)!/2^n for n<=4. %Y A177284 Cf. A000680, A177283. %K A177284 nonn %O A177284 0,3 %A A177284 _R. H. Hardin_, May 06 2010 %E A177284 a(0), a(13) from _Alois P. Heinz_, Jan 14 2016 %E A177284 Terms a(14) and beyond from _Andrew Howroyd_, May 14 2020