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A279568
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Number of length n inversion sequences avoiding the patterns 110, 120, 201, and 210.
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23
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1, 1, 2, 6, 22, 90, 396, 1833, 8801, 43441, 219092, 1124201, 5850414, 30805498, 163824559, 878655117, 4747341879, 25815026491, 141173582016, 775920816789, 4283833709457, 23746640019657, 132116647765569, 737485227605338, 4129174120158569, 23183379592361839
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OFFSET
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0,3
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COMMENTS
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A length n inversion sequence e_1e_2...e_n is a sequence of integers where 0 <= e_i <= i-1. The term a(n) counts those length n inversion sequences with no entries e_i, e_j, e_k (where i<j<k) such that e_j <> e_k and e_i > e_k. This is the same as the set of length n inversion sequences avoiding 110, 120, 201, and 210.
It was shown that a_n also counts those length n inversion sequences with no entries e_i, e_j, e_k (where i<j<k) such that e_i <> e_j and e_i > e_k. This is the same as the set of length n inversion sequences avoiding 100, 120, 201, and 210.
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LINKS
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FORMULA
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a(n) ~ c * d^n / n^(3/2), where d = 5.98041772076926677236919875200507... is the positive root of the equation -32 - 195*d - 12*d^2 - 112*d^3 + 20*d^4 = 0 and c = 0.1056946795054351807407212356928404107733262398133039312067247126343... - Vaclav Kotesovec, Oct 07 2021
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EXAMPLE
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The length 4 inversion sequences avoiding (110, 120, 201, 210) are 0000, 0001, 0002, 0003, 0010, 0011, 0012, 0013, 0020, 0021, 0022, 0023, 0100, 0101, 0102, 0103, 0111, 0112, 0113, 0121, 0122, 0123.
The length 4 inversion sequences avoiding (100, 120, 201, 210) are 0000, 0001, 0002, 0003, 0010, 0011, 0012, 0013, 0020, 0021, 0022, 0023, 0101, 0102, 0103, 0110, 0111, 0112, 0113, 0121, 0122, 0123.
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MAPLE
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b:= proc(n, i, l) option remember; `if`(n=0, 1, add((h->
b(n-1, i-h+2, j-h+1))(max(1, `if`(j=l, 0, l))), j=1..i))
end:
a:= n-> b(n, 1$2):
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MATHEMATICA
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b[n_, i_, l_] := b[n, i, l] = If[n == 0, 1, Sum[b[n-1, i-#+2, j-#+1]& @ Max[1, If[j == l, 0, l]], {j, 1, i}]]; a[n_] := b[n, 1, 1]; Table[a[n], {n, 0, 30}] (* Jean-François Alcover, Jul 10 2017, after Alois P. Heinz *)
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CROSSREFS
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Cf. A000108, A057552, A263777, A263778, A263779, A263780, A279551, A279552, A279553, A279554, A279555, A279556, A279557, A279558, A279559, A279560, A279561, A279562, A279563, A279564, A279565, A279566, A279567, A279569, A279570, A279571, A279572, A279573.
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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