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A026269
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a(n) = number of (s(0), s(1), ..., s(n)) such that every s(i) is a nonnegative integer, s(0) = 0 = s(n), s(1) = 1, |s(i) - s(i-1)| <= 1 for i >= 2, |s(2) - s(1)| = 1, |s(3) - s(2)| = 1 if s(2) = 1. Also a(n) = T(n,n) and a(n) = Sum{T(k,k-1)}, k = 1,2,...,n, where T is array in A026268.
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2
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1, 2, 4, 10, 25, 64, 166, 436, 1157, 3098, 8360, 22714, 62086, 170614, 471096, 1306374, 3636708, 10159590, 28473132, 80032638, 225562929, 637301652, 1804751718, 5121677512, 14563448593, 41487279622, 118389089432, 338381552294, 968627180975
(list; graph; refs; listen; history; internal format)
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OFFSET
| 2,2
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FORMULA
| G.f.: 4z^2(1-z^2)/[1-z+sqrt(1-2z-3z^2)]^2.
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MATHEMATICA
| Drop[CoefficientList[Series[4x^2(1-x^2)/(1-x+Sqrt[1-2x-3x^2])^2, {x, 0, 30}], x], 2] (* From Harvey P. Dale, May 05 2011 *)
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CROSSREFS
| First differences of A102071.
Sequence in context: A173610 A036887 A151536 * A000645 A005958 A166516
Adjacent sequences: A026266 A026267 A026268 * A026270 A026271 A026272
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KEYWORD
| nonn
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AUTHOR
| Clark Kimberling (ck6(AT)evansville.edu)
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EXTENSIONS
| More terms from R. Stephan, Dec 30 2004
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