A370390 Number of permutations of [n] whose longest block is of length 2. A block of a permutation is a maximal sequence of consecutive integers which appear in consecutive positions.

0, 0, 1, 2, 10, 53, 334, 2428, 20009, 184440, 1881050, 21034905, 255967940, 3367720736, 47641219569, 721160081974, 11631770791362, 199159952915293, 3607908007376418, 68946510671942892, 1386140583681969289, 29247292475233307612, 646231776371742321826

Offset: 0

Author

Seiichi Manyama, Feb 17 2024

Keywords

• nonn

Links

• Alois P. Heinz, Table of n, a(n) for n = 0..450

Crossrefs

Column k=2 of A184182.

Cf. A000255, A002628.

Programs

PARI

my(N=30, x='x+O('x^N)); concat([0, 0], Vec(sum(k=0, N, k!*x^k*(((1-x^2)/(1-x^3))^k-((1-x)/(1-x^2))^k))))

Formula

a(n) = A002628(n) - A000255(n-1).

G.f.: Sum_{k>=0} k! * x^k * ( ((1-x^2)/(1-x^3))^k - ((1-x)/(1-x^2))^k ).