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Number of permutations of 2 copies of 1..n with all adjacent differences <= 9 in absolute value.
0

%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