A177284 - OEIS

%I #10 May 17 2020 01:41:51

%S 1,1,6,90,2520,41580,516180,6068622,76331906,958679970,11679900408,

%T 138047313960,1610654864328,18649754961744,214256589488616,

%U 2439692058769566,27562317214408488,309367132582535226,3453299423388485028,38354816922190327314,424048220090513056908

%N Number of permutations of 2 copies of 1..n with all adjacent differences <= 3 in absolute value.

%C a(n) = (2n)!/2^n for n<=4.

%H Andrew Howroyd, <a href="/A177284/b177284.txt">Table of n, a(n) for n = 0..200</a>

%Y Cf. A000680, A177283.

%K nonn

%O 0,3

%A _R. H. Hardin_, May 06 2010

%E a(0), a(13) from _Alois P. Heinz_, Jan 14 2016

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