A177283 - OEIS

%I #15 May 30 2025 16:57:40

%S 1,1,6,90,660,3846,24326,148146,850920,4788114,26548704,144707622,

%T 778116870,4140736398,21836757474,114262071558,593931441790,

%U 3069689137294,15786994900896,80840741811238,412406022128206,2096937750144962,10631278302356284,53761972675284874

%N Number of permutations of 2 copies of 1..n with all adjacent differences <= 2 in absolute value.

%H Andrew Howroyd, <a href="/A177283/b177283.txt">Table of n, a(n) for n = 0..200</a>

%F a(n) = (2n)!/2^n for n<=3.

%Y Cf. A000680.

%K nonn

%O 0,3

%A _R. H. Hardin_, May 06 2010

%E Terms a(16) and beyond from _Andrew Howroyd_, May 14 2020