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A328436 Number of inversion sequences of length n avoiding the consecutive patterns 000 and 001.

%I A328436 #12 Jun 01 2022 01:55:48
%S A328436 1,1,2,3,9,37,190,1181,8564,70914,659810,6811371,77232836,953969548,
%T A328436 12747856402,183218649413,2818050980941,46182485773217,
%U A328436 803323102085452,14781372445602234,286838921699435184,5854404018902152208,125367868007259046305,2810511319383912299122
%N A328436 Number of inversion sequences of length n avoiding the consecutive patterns 000 and 001.
%C A328436 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 001.
%H A328436 Juan S. Auli and Sergi Elizalde, <a href="https://arxiv.org/abs/1906.07365">Consecutive patterns in inversion sequences II: avoiding patterns of relations</a>, arXiv:1906.07365 [math.CO], 2019.
%e A328436 The a(4)=9 length 4 inversion sequences avoiding the consecutive patterns 000 and 001 are 0100, 0110, 0120, 0101, 0121, 0102, 0122, 0103, and 0123.
%p A328436 # after _Alois P. Heinz_ in A328357
%p A328436 b := proc(n, x, t) option remember; `if`(n = 0, 1, add(
%p A328436        `if`(t and i = x, 0, b(n - 1, i, i <= x)), i = 0 .. n - 1))
%p A328436      end proc:
%p A328436 a := n -> b(n, -1, false):
%p A328436 seq(a(n), n = 0 .. 24);
%t A328436 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}]];
%t A328436 a[n_] := b[n, -1, False];
%t A328436 a /@ Range[0, 24] (* _Jean-François Alcover_, Mar 02 2020 after _Alois P. Heinz_ in A328357 *)
%Y A328436 Cf. A328357, A328358, A328429, A328430, A328431, A328432, A328433, A328434, A328435, A328437, A328438, A328439, A328440, A328441, A328442.
%K A328436 nonn
%O A328436 0,3
%A A328436 _Juan S. Auli_, Oct 17 2019