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A026124 a(n) = number of (s(0),s(1),...,s(n)) such that every s(i) is a nonnegative integer, s(0) = 1, s(n) = 3, |s(1) - s(0)| = 1, |s(i) - s(i-1)| <= 1 for i >= 2. Also a(n) = T(n,n-2), where T is the array in A026120. 3
1, 2, 7, 20, 59, 170, 489, 1400, 4002, 11428, 32626, 93160, 266136, 760800, 2176644, 6232896, 17864841, 51253794, 147188535, 423098404, 1217371023, 3505992050, 10106384621, 29158627592, 84200265555, 243345531806, 703858089717 (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-z)^2M^4, with M the g.f. of the Motzkin numbers (A001006).

Conjecture: (n+6)*a(n) +(-5*n-19)*a(n-1) +4*n*a(n-2) +8*(n+1)*a(n-3) +(-5*n+22)*a(n-4) +3*(-n+5)*a(n-5)=0. - R. J. Mathar, Jun 23 2013

CROSSREFS

First differences of A026109.

Sequence in context: A292400 A007460 A034899 * A026153 A025180 A201967

Adjacent sequences:  A026121 A026122 A026123 * A026125 A026126 A026127

KEYWORD

nonn

AUTHOR

Clark Kimberling

STATUS

approved

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Last modified May 11 21:35 EDT 2021. Contains 343808 sequences. (Running on oeis4.)