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”).
%I #9 Feb 10 2015 06:54:44
%S 1,3,12,50,240,1281,7588,49392,350775,2700115,22399476,199258488,
%T 1892343362,19111149225,204532683600,2312443695920,27541725738255,
%U 344681838284220,4522200516582650,62068382381998440,889492878311173290,13286058811968721515
%N Number of ascent sequences of length n with exactly two flat steps.
%H Joerg Arndt and Alois P. Heinz, <a href="/A242155/b242155.txt">Table of n, a(n) for n = 3..140</a>
%F a(n) ~ Pi^(3/2)/(2*sqrt(3)*exp(Pi^2/12)) * (6/Pi^2)^n * n! * sqrt(n). - _Vaclav Kotesovec_, Aug 27 2014
%t 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 *)
%Y Column k=2 of A242153.
%K nonn
%O 3,2
%A _Joerg Arndt_ and _Alois P. Heinz_, May 05 2014