%I #11 May 25 2015 19:41:56
%S 1,1,2,6,24,120,480,3600,30240,151200,1693440,20321280,121927680,
%T 1828915200,28740096000,201180672000,3793692672000,73977007104000,
%U 591816056832000,13463815292928000,314155690168320000
%N Number of permutations of {1,2,3,...,n} such that no multiples of 3 are adjacent.
%F a(n) = factorial(n-floor(n/3))*fallfac(n+1-floor(n/3),floor(n/3)), where fallfac is falling factorial, A008279.
%F a(n) ~ n! * 2^(2*n+2-2*floor(n,3)/3) / 3^(n+1/2). - _Vaclav Kotesovec_, May 23 2015
%t Table[Factorial[n-Floor[n/3]]*FactorialPower[n+1-Floor[n/3],Floor[n/3],1], {n, 0, 20}]
%Y Cf. A008279, A010790, A258213.
%K nonn,easy
%O 0,3
%A _Ran Pan_, May 23 2015
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