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A328438 Number of inversion sequences of length n avoiding the consecutive patterns 000 and 011.

%I A328438 #12 Jun 01 2022 01:56:01
%S A328438 1,1,2,4,13,57,304,1937,14315,120264,1131896,11794453,134774963,
%T A328438 1675630582,22516745452,325188337067,5022796990606,82620491929333,
%U A328438 1441894214312037,26609607869036180,517741915593936360,10592513721179374467,227325651424365263577,5106351205789851629476
%N A328438 Number of inversion sequences of length n avoiding the consecutive patterns 000 and 011.
%C A328438 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.
%H A328438 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 A328438 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.
%p A328438 # after _Alois P. Heinz_ in A328357
%p A328438 b := proc(n, x, t) option remember; `if`(n = 0, 1, add(
%p A328438        `if`(t and i <= x, 0, b(n - 1, i, i = x)), i = 0 .. n - 1))
%p A328438      end proc:
%p A328438 a := n -> b(n, -1, false):
%p A328438 seq(a(n), n = 0 .. 24);
%t A328438 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 A328438 a[n_] := b[n, -1, False];
%t A328438 a /@ Range[0, 24] (* _Jean-François Alcover_, Mar 02 2020 after _Alois P. Heinz_ in A328357 *)
%Y A328438 Cf. A328357, A328358, A328429, A328430, A328431, A328432, A328433, A328434, A328435, A328436, A328437, A328439, A328440, A328441, A328442.
%K A328438 nonn
%O A328438 0,3
%A A328438 _Juan S. Auli_, Oct 17 2019