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%I #8 May 17 2020 01:41:45
%S 1,1,6,90,2520,113400,3356640,75336660,1500131820,29660400822,
%T 620330067782,13125403608162,274194614811528,5605288189594200,
%U 112437639146871360,2235073249396108440,44209749453949113840,870798759948756386976,17069437222534487122608,332789644153015982087550,6455422726721469064982952
%N Number of permutations of 2 copies of 1..n with all adjacent differences <= 4 in absolute value.
%C a(n) = (2n)!/2^n for n<=5.
%Y Cf. A177283.
%K nonn
%O 0,3
%A _R. H. Hardin_, May 06 2010
%E a(0)=1 prepended and terms a(13) and beyond from _Andrew Howroyd_, May 14 2020
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