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Search: id:A026124
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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. +0
4
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; listen)
OFFSET

2,2

FORMULA

G.f.: z^2(1-z)^2M^4, with M the g.f. of the Motzkin numbers (A001006).

CROSSREFS

First differences of A026109.

Sequence in context: A094982 A007460 A034899 this_sequence A026153 A025180 A116950

Adjacent sequences: A026121 A026122 A026123 this_sequence A026125 A026126 A026127

KEYWORD

nonn

AUTHOR

Clark Kimberling (ck6(AT)evansville.edu)

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Last modified August 28 22:44 EDT 2008. Contains 143251 sequences.


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