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A026123 a(n) = number of (s(0),s(1),...,s(n)) such that every s(i) is a nonnegative integer, s(0) = 1, s(n) = 2, |s(1) - s(0)| = 1, |s(i) - s(i-1)| <= 1 for i >= 2. Also a(n) = T(n,n-1), where T is the array in A026120; a(n) = U(n,n+1), where U is the array in A026148. 3
1, 4, 10, 28, 76, 209, 575, 1589, 4405, 12253, 34189, 95679, 268503, 755457, 2130717, 6023235, 17063139, 48434514, 137741280, 392407134, 1119766942, 3200326627, 9160055809, 26254474379, 75348899051, 216515177336, 622887159274 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

LINKS

Table of n, a(n) for n=2..28.

FORMULA

G.f.: z^2(-1+(1-z)^2M^3), with M the g.f. of the Motzkin numbers (A001006).

Conjecture: (n+5)*a(n) +5*(-n-3)*a(n-1) +(5*n+1)*a(n-2) +(5*n+3)*a(n-3) +6*(-n+3)*a(n-4)=0. - R. J. Mathar, Jun 23 2013

CROSSREFS

First differences of A026134.

Sequence in context: A111308 A121302 A026150 * A091468 A103457 A083587

Adjacent sequences:  A026120 A026121 A026122 * A026124 A026125 A026126

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified February 25 05:15 EST 2020. Contains 332217 sequences. (Running on oeis4.)