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A328429
Number of inversion sequences of length n avoiding the consecutive patterns 012, 101, 102, and 201.
15
1, 1, 2, 5, 14, 46, 170, 691, 3073, 14809, 76666, 423886, 2490514, 15479614, 101389508, 697513653, 5025406212, 37819960947, 296618360520, 2419362514273, 20484053318220, 179723185666151, 1631519158000420, 15302546831928727, 148099068509673563
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 012, 101, 102, and 201.
LINKS
EXAMPLE
The a(4)=14 length 4 inversion sequences avoiding the consecutive patterns 012, 101, 102, and 201 are 0000, 0100, 0010, 0110, 0020, 0001, 0011, 0111, 0021, 0002, 0112, 0022, 0003, and 0113.
MAPLE
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-François Alcover, Mar 02 2020 after Alois P. Heinz in A328357 *)
KEYWORD
nonn
AUTHOR
Juan S. Auli, Oct 15 2019
STATUS
approved