login

Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).

A242155
Number of ascent sequences of length n with exactly two flat steps.
3
1, 3, 12, 50, 240, 1281, 7588, 49392, 350775, 2700115, 22399476, 199258488, 1892343362, 19111149225, 204532683600, 2312443695920, 27541725738255, 344681838284220, 4522200516582650, 62068382381998440, 889492878311173290, 13286058811968721515
OFFSET
3,2
LINKS
Joerg Arndt and Alois P. Heinz, Table of n, a(n) for n = 3..140
FORMULA
a(n) ~ Pi^(3/2)/(2*sqrt(3)*exp(Pi^2/12)) * (6/Pi^2)^n * n! * sqrt(n). - Vaclav Kotesovec, Aug 27 2014
MATHEMATICA
b[n_, i_, t_] := b[n, i, t] = If[n == 0, 1, Expand[Sum[ If[j == i, x, 1]*b[n - 1, j, t + If[j > i, 1, 0]], {j, 0, t + 1}]]]; a[n_] := Coefficient[b[n, -1, -1], x, 2]; Table[a[n], {n, 3, 30}] (* Jean-François Alcover, Feb 10 2015, after A242153 *)
CROSSREFS
Column k=2 of A242153.
Sequence in context: A268650 A151181 A094601 * A009024 A265083 A043291
KEYWORD
nonn
AUTHOR
Joerg Arndt and Alois P. Heinz, May 05 2014
STATUS
approved