A340567 Total number of ascents in all faro permutations of length n.

0, 0, 1, 4, 11, 26, 62, 134, 303, 634, 1394, 2872, 6206, 12676, 27068, 54994, 116423, 235706, 495722, 1001168, 2094714, 4223020, 8798756, 17715084, 36782246, 73980516, 153161332, 307808464, 635675228, 1276699336, 2630957432, 5281304554, 10863149303, 21797013946

Offset

0,4

Comments

Faro permutations are permutations avoiding the three consecutive patterns 231, 321 and 312. They are obtained by a perfect faro shuffle of two nondecreasing words of lengths differing by at most one.

Links

Table of n, a(n) for n=0..33.

Jean-Luc Baril, Alexander Burstein, and Sergey Kirgizov, Pattern statistics in faro words and permutations, arXiv:2010.06270 [math.CO], 2020. See Table 1.

Formula

G.f.: 2*x*(4*x^2 + x + sqrt(1 - 4*x^2) - 1)/((1 - 2*x)*sqrt(1 - 4*x^2)*(sqrt(1 - 4*x^2) + 1)).

Example

For n = 3 there are 3 faro permutations, namely 123, 213, 132. They contain 4 ascents (12, 23, 13 and 13) in total.

Prog

(PARI) seq(n)={my(t=sqrt(1-4*x^2+O(x^n))); Vec(2*x*(4*x^2 + x + t - 1)/((1 - 2*x)*t*(t + 1)), -(1+n))} \\ Andrew Howroyd, Jan 11 2021

Crossrefs

A001405 counts faro permutations of length n.

Cf. A107373 (descents), A340568, A340569.

Sequence in context: A034334 A036891 A183276 * A268775 A294840 A014630

Adjacent sequences: A340564 A340565 A340566 * A340568 A340569 A340570

Keyword

nonn

Author

Sergey Kirgizov, Jan 11 2021

Status

approved