A026270 Number of (s(0), s(1), ..., s(n)) such that every s(i) is a nonnegative integer, s(0) = 0, s(1) = 1 = s(n), |s(i) - s(i-1)| <= 1 for i >= 2, |s(2) - s(1)| = 1, |s(3) - s(2)| = 1 if s(2) = 1. Also T(n,n-1), where T is the array in A026268.

1, 2, 6, 15, 39, 102, 270, 721, 1941, 5262, 14354, 39372, 108528, 300482, 835278, 2330334, 6522882, 18313542, 51559506, 145530291, 411738723, 1167450066, 3316925794, 9441771081, 26923831029, 76901809810, 219992462862, 630245628681, 1808029517585

Offset

2,2

Links

Table of n, a(n) for n=2..30.

Formula

G.f.: -1 + 4z^2(1-z)(1-z^2)/[1-z+sqrt(1-2z-3z^2)]^2.

Conjecture: (n+3)*a(n) +3*(-n-1)*a(n-1) +(-n-1)*a(n-2) +3*(n-5)*a(n-3)=0. - R. J. Mathar, Jun 23 2013

Crossrefs

First differences of A026269. Pairwise sums of A026122.

Sequence in context: A094969 A001674 A121331 * A321646 A246563 A259399

Adjacent sequences: A026267 A026268 A026269 * A026271 A026272 A026273

Keyword

nonn

Author

Clark Kimberling

Status

approved