OFFSET
1,1
COMMENTS
Define a circular k-succession in a permutation p on [n] as either a pair p(i),p(i+1) if p(i+1)=p(i)+k, or as the pair p(n),p(1) if p(1)=p(n)+k. If we let d*(n,k) be the number of permutations on [n] that avoid substrings (j,j+k), 1 <= j <= n, k=3, i.e., permutations with no circular 3-succession, then a(n) counts d*(n+3,3).
For example, for n=1, the permutations in S4 that contain the substring {14} in circular 3-succession are 1423, 1432, 2143, 2314, 3142, 3214, 4231, 4321, therefore d*(4,3) consists of the complementary permutations in S4, and a(1)=16.
LINKS
Enrique Navarrete, Generalized K-Shift Forbidden Substrings in Permutations, arXiv:1610.06217 [math.CO], 2016.
FORMULA
a(n) = (n+3)* Sum_{j=0..n} (-1)^j*binomial(n,j)*(n-j+2)!.
Conjecture: a(n) = (n+3)*A055790(n+1). - R. J. Mathar, Jul 15 2017
EXAMPLE
a(2)=70 since there are 70 permutations in S5 with no circular 3-succession, i.e., permutations that avoid substrings {14,25} such as 25134 or 51342.
MAPLE
A284844 := proc(n)
local j;
add( (-1)^j*binomial(n, j)*(n-j+2)!, j=0..n) ;
%*(n+3) ;
end proc:
seq(A284844(n), n=1..20) ; # R. J. Mathar, Jul 15 2017
MATHEMATICA
a[n_] := ((n+3)*((n*(n+5)+5)*Subfactorial[n+2]+(-1)^(n+1)*(n+1)))/((n+2)*(n+1));
Array[a, 20] (* Jean-François Alcover, Dec 09 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Enrique Navarrete, Apr 03 2017
STATUS
approved