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A328438 Number of inversion sequences of length n avoiding the consecutive patterns 000 and 011. 15
1, 1, 2, 4, 13, 57, 304, 1937, 14315, 120264, 1131896, 11794453, 134774963, 1675630582, 22516745452, 325188337067, 5022796990606, 82620491929333, 1441894214312037, 26609607869036180, 517741915593936360, 10592513721179374467, 227325651424365263577, 5106351205789851629476 (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 000 and 011.

LINKS

Table of n, a(n) for n=0..23.

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

EXAMPLE

The a(4)=13 length 4 inversion sequences avoiding the consecutive patterns 000 and 011 are 0100, 0010, 0020, 0120, 0101, 0021, 0121, 0102, 0012, 0103, 0013, 0023, and 0123.

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)), 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]], {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, A328434, A328435, A328436, A328437, A328439, A328440, A328441, A328442.

Sequence in context: A030968 A030862 A030923 * A039831 A173184 A111977

Adjacent sequences:  A328435 A328436 A328437 * A328439 A328440 A328441

KEYWORD

nonn

AUTHOR

Juan S. Auli, Oct 17 2019

STATUS

approved

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Last modified October 21 19:16 EDT 2021. Contains 348155 sequences. (Running on oeis4.)