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 A177283 #15 May 30 2025 16:57:40 %S A177283 1,1,6,90,660,3846,24326,148146,850920,4788114,26548704,144707622, %T A177283 778116870,4140736398,21836757474,114262071558,593931441790, %U A177283 3069689137294,15786994900896,80840741811238,412406022128206,2096937750144962,10631278302356284,53761972675284874 %N A177283 Number of permutations of 2 copies of 1..n with all adjacent differences <= 2 in absolute value. %H A177283 Andrew Howroyd, <a href="/A177283/b177283.txt">Table of n, a(n) for n = 0..200</a> %F A177283 a(n) = (2n)!/2^n for n<=3. %Y A177283 Cf. A000680. %K A177283 nonn %O A177283 0,3 %A A177283 _R. H. Hardin_, May 06 2010 %E A177283 Terms a(16) and beyond from _Andrew Howroyd_, May 14 2020