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A279563
Number of length n inversion sequences avoiding the patterns 102, 201, and 210.
23
1, 1, 2, 6, 22, 85, 328, 1253, 4754, 17994, 68158, 258808, 985906, 3768466, 14451386, 55585014, 214377618, 828795169, 3211030684, 12464308997, 48465092366, 188733879657, 735977084412, 2873525548315, 11231884145434, 43947466923095, 172115939825516
OFFSET
0,3
COMMENTS
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_i > e_j <> e_k and e_i <> e_k. This is the same as the set of length n inversion sequences avoiding 102, 201, and 210.
LINKS
Megan A. Martinez, Carla D. Savage, Patterns in Inversion Sequences II: Inversion Sequences Avoiding Triples of Relations, arXiv:1609.08106 [math.CO], 2016.
FORMULA
a(n) ~ 4^n / (3*sqrt(Pi*n)). - Vaclav Kotesovec, Oct 07 2021
EXAMPLE
The length 4 inversion sequences avoiding (102, 201, 210) are 0000, 0001, 0002, 0003, 0010, 0011, 0012, 0013, 0020, 0021, 0022, 0023, 0100, 0101, 0110, 0111, 0112, 0113, 0120, 0121, 0122, 0123.
MAPLE
a:= proc(n) option remember; `if`(n<4, n!,
((2*(12*n^3-91*n^2+213*n-149))*a(n-1)
-(3*(21*n^3-162*n^2+392*n-291))*a(n-2)
+(2*(33*n^3-257*n^2+633*n-484))*a(n-3)
-(4*(2*n-7))*(3*n^2-13*n+13)*a(n-4))
/ ((n-1)*(3*n^2-19*n+29)))
end:
seq(a(n), n=0..30); # Alois P. Heinz, Feb 22 2017
MATHEMATICA
a[n_] := a[n] = If[n < 4, n!, ((2*(12*n^3 - 91*n^2 + 213*n - 149))*a[n-1] - (3*(21*n^3 - 162*n^2 + 392*n - 291))*a[n-2] + (2*(33*n^3 - 257*n^2 + 633*n - 484))*a[n-3] - (4*(2*n - 7))*(3*n^2 - 13*n + 13)*a[n-4]) / ((n - 1)*(3*n^2 - 19*n + 29))]; Array[a, 30, 0] (* Jean-François Alcover, Nov 06 2017, after Alois P. Heinz *)
KEYWORD
nonn
AUTHOR
Megan A. Martinez, Feb 09 2017
EXTENSIONS
a(10)-a(26) from Alois P. Heinz, Feb 22 2017
STATUS
approved