A243475 Number of isoscent sequences of length n with exactly two descents.

2, 28, 241, 1667, 10142, 56748, 299485, 1514445, 7415873, 35424718, 165963977, 765639729, 3488740303, 15739196772, 70434524941, 313136541466, 1384706596078, 6096641331963, 26747810596747, 117015840972949, 510745056446585, 2225207050547044, 9680854941464722

Offset

5,1

Links

Joerg Arndt and Alois P. Heinz, Table of n, a(n) for n = 5..100

Example

a(5) = 2: [0,0,2,1,0], [0,1,0,1,0].

Mathematica

b[n_, i_, t_] := b[n, i, t] = If[n<1, 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, 0, 0], x, 2]; Table[a[n], {n, 5, 40}] (* Jean-François Alcover, Feb 09 2015, after A242352 *)

Crossrefs

Column k=2 of A242352.

Sequence in context: A281124 A244721 A001759 * A019441 A147537 A183067

Adjacent sequences: A243472 A243473 A243474 * A243476 A243477 A243478

Keyword

nonn

Author

Joerg Arndt and Alois P. Heinz, Jun 05 2014

Status

approved