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A328434 Number of inversion sequences of length n avoiding the consecutive patterns 101, 102, 201, and 210. 15
1, 1, 2, 6, 21, 81, 346, 1630, 8350, 45958, 269815, 1681285, 11071336, 76743040, 558062437, 4244853573, 33687390663, 278296576327, 2388351295760, 21254019548162, 195801111412320, 1864508416302520, 18326903140310011, 185711672802101781, 1937795878138303715 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

A length n inversion sequence e_1e_2...e_n is a sequence of integers such that 0 <= e_i < i. The term a(n) counts the inversion sequences of length n with no entries e_i, e_{i+1}, e_{i+2} such that e_i > e_{i+1} != e_{i+2}. This is the same as the set of inversion sequences of length n avoiding the consecutive patterns 101, 102, 201, and 210.

LINKS

Table of n, a(n) for n=0..24.

Juan S. Auli and Sergi Elizalde, Consecutive patterns in inversion sequences II: avoiding patterns of relations, arXiv:1906.07365 [math.CO], 2019.

EXAMPLE

Note that a(4)=21. Indeed, of the 24 inversion sequences of length 4, the only ones that do not avoid the consecutive patterns 101, 102, 201, and 210 are 0101, 0102 and 0103.

MAPLE

# after Alois P. Heinz in A328357

b := proc(n, x, t) option remember; `if`(n=0, 1, add(

       `if`(t and i>x, 0, b(n-1, i, i<>x and x>-1)), i=0..n-1))

     end proc:

a := n -> b(n, -1, false):

seq(a(n), n = 0 .. 24);

MATHEMATICA

b[n_, x_, t_] := b[n, x, t] = If[n == 0, 1, Sum[If[t && i > x, 0, b[n - 1, i, i != x && x > -1]], {i, 0, n - 1}]];

a[n_] := b[n, -1, False];

a /@ Range[0, 24] (* Jean-Fran├žois Alcover, Mar 02 2020 after _Alois P.Heinz_ in A328357 *)

CROSSREFS

Cf. A328357, A328358, A328429, A328430, A328431, A328432, A328433, A328435, A328436, A328437, A328438, A328439, A328440, A328441, A328442.

Sequence in context: A279565 A150214 A150215 * A150216 A150217 A150218

Adjacent sequences:  A328431 A328432 A328433 * A328435 A328436 A328437

KEYWORD

nonn

AUTHOR

Juan S. Auli, Oct 16 2019

STATUS

approved

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Last modified July 27 02:39 EDT 2021. Contains 346302 sequences. (Running on oeis4.)