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A026327
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a(n) = number of (s(0), s(1), ..., s(n)) such that s(i) is a nonnegative integer and |s(i) - s(i-1)| <= 1 for i = 1,2,...,n, s(0) = 2, s(n) = 4. Also a(n) = T(n,n-2), where T is the array in A026323.
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3
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1, 3, 10, 30, 90, 266, 783, 2295, 6710, 19580, 57057, 166101, 483210, 1405080, 4084590, 11872494, 34508997, 100313635, 291646580, 848102640, 2466916474, 7177785582, 20891443950, 60827142350, 177167486925, 516217883571, 1504692189588
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OFFSET
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2,2
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COMMENTS
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Number of paths in the plane x>=0 and y>=-2, from (0,0) to (n,2), and consisting of steps U=(1,1), D=(1,-1) and H=(1,0). For example, for n=4, we have the 10 paths: UUUD, UUHH, UUDU, UHUH, UHHU, UDUU, HUUH, HUHU, HHUU, DUUU. - José Luis Ramírez Ramírez, Apr 20 2015
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LINKS
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FORMULA
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Conjecture: -(n+8)*(n-2)*a(n) +3*(2*n^2+7*n-28)*a(n-1) +3*(-3*n^2-n+36)*a(n-2) -4*(n+4)*(n-1)*a(n-3) +12*(n-1)*(n-2)*a(n-4)=0. - R. J. Mathar, Jun 23 2013
G.f: x^2*M(x)^2/(1-x-x^2*(M(x)+1/(1-x-x^2/(1-x)))), where M(x) is g.f. of Motzkin paths. - José Luis Ramírez Ramírez, Apr 20 2015
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MATHEMATICA
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Rest[Rest[CoefficientList[Series[x^2*((1-x-Sqrt[1-2*x-3*x^2])/(2*x^2))^2/(1-x-x^2*((1-x-Sqrt[1-2*x-3*x^2])/(2*x^2)+1/(1-x-x^2/(1-x)))), {x, 0, 20}], x]]] (* Vaclav Kotesovec, Apr 21 2015 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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