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A180187 Number of successions in all the permutations p of [n] such that p(1)=1 and having no 3-sequences. A succession of a permutation p is a position i such that p(i +1) - p(i) = 1. 3
0, 1, 0, 3, 14, 72, 468, 3453, 28782, 267831, 2752828, 30984336, 379125192, 5011756625, 71190365580, 1081514329155, 17499480412746, 300473929597320, 5457031426340748, 104520033700333069, 2105651342251571562 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,4

COMMENTS

a(n) = Sum(k*A180186(n,k), k>=0).

Contribution from Emeric Deutsch, Sep 07 2010: (Start)

a(n) is also the number of fixed points in all those permutations of [n-1] that have no adjacent fixed points. Example: a(4)=3 because in 132, 213, 231, 312, 321 we have 1+1+0+0+1 fixed points.

a(n) is also the number of permutations of [n] having exactly 1 pair of adjacent fixed points. Example: a(4)=3 because we have 1243, 4231, and 2134.

(End)

REFERENCES

Wayne M. Dymacek, Isaac Lambert and Kyle Parsons, Arithmetic Progressions in Permutations, http://math.ku.edu/~ilambert/CN.pdf, 2012. - From N. J. A. Sloane, Sep 15 2012

LINKS

Table of n, a(n) for n=1..21.

FORMULA

a(n)=Sum(k*binom(n-k,k)*d(n-1-k), k=0..floor(n/2)), where d(j)=A000166(j) are the derangement numbers.

EXAMPLE

a(4)=3 because in 1*243, 1324, 13*42, 142*3, 1432 we have 3 successions (marked *).

MAPLE

d[0] := 1: for n to 51 do d[n] := n*d[n-1]+(-1)^n end do: seq(sum(k*binomial(n-k, k)*d[n-1-k], k = 0 .. floor((1/2)*n)), n = 1 .. 22);

MATHEMATICA

a[0] = 1; a[n_] := a[n] = n*a[n - 1] + (-1)^n; f[n_] := Sum[k*Binomial[n - k, k]*a[n - k - 1], {k, 0, n/2}]; Array[f, 21] (* Robert G. Wilson v, Apr 01 2011 *)

CROSSREFS

Cf. A000166, A180186.

A column of A216718. - N. J. A. Sloane, Sep 15 2012

Sequence in context: A098648 A026295 A118650 * A080238 A213228 A277939

Adjacent sequences:  A180184 A180185 A180186 * A180188 A180189 A180190

KEYWORD

nonn

AUTHOR

Emeric Deutsch, Sep 06 2010

STATUS

approved

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Last modified June 25 06:13 EDT 2017. Contains 288709 sequences.