%I #8 Jan 05 2022 06:43:56
%S 1,1,6,90,2520,113400,7484400,681080400,81729648000,12504636144000,
%T 2375880867360000,367786358267328000,47683626117839712000
%N Number of permutations of 2 copies of 1..n with all adjacent differences <= 9 in absolute value.
%F a(n) = (2n)!/2^n for n<=10.
%Y Cf. A000680.
%K nonn,more
%O 0,3
%A _R. H. Hardin_, May 06 2010
%E a(0)=1 prepended and a(12) added by _Alois P. Heinz_, Jan 05 2022