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A258216 Number of permutations of {1,2,3,...,n} such that no multiples of 3 are adjacent. 0
1, 1, 2, 6, 24, 120, 480, 3600, 30240, 151200, 1693440, 20321280, 121927680, 1828915200, 28740096000, 201180672000, 3793692672000, 73977007104000, 591816056832000, 13463815292928000, 314155690168320000 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

LINKS

Table of n, a(n) for n=0..20.

FORMULA

a(n) = factorial(n-floor(n/3))*fallfac(n+1-floor(n/3),floor(n/3)), where fallfac is falling factorial, A008279.

a(n) ~ n! * 2^(2*n+2-2*floor(n,3)/3) / 3^(n+1/2). - Vaclav Kotesovec, May 23 2015

MATHEMATICA

Table[Factorial[n-Floor[n/3]]*FactorialPower[n+1-Floor[n/3], Floor[n/3], 1], {n, 0, 20}]

CROSSREFS

Cf. A008279, A010790, A258213.

Sequence in context: A174701 A178848 A173845 * A263748 A072856 A228401

Adjacent sequences:  A258213 A258214 A258215 * A258217 A258218 A258219

KEYWORD

nonn,easy

AUTHOR

Ran Pan, May 23 2015

STATUS

approved

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Last modified September 26 06:54 EDT 2017. Contains 292502 sequences.