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A182895
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Number of (1,0)-steps at level 0 in all weighted lattice paths in L_n.
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3
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0, 1, 3, 7, 19, 50, 130, 341, 893, 2337, 6119, 16020, 41940, 109801, 287463, 752587, 1970299, 5158310, 13504630, 35355581, 92562113, 242330757, 634430159, 1660959720, 4348449000, 11384387281, 29804712843, 78029751247, 204284540899
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OFFSET
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0,3
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COMMENTS
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The members of L_n are paths of weight n that start at (0,0) and end on the horizontal axis and whose steps are of the following four kinds: a (1,0)-step with weight 1, a (1,0)-step with weight 2, a (1,1)-step with weight 2, and a (1,-1)-step with weight 1. The weight of a path is the sum of the weights of its steps.
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LINKS
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FORMULA
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G.f.: z(1+z)/[(1+z+z^2)(1-3z+z^2)].
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EXAMPLE
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a(3) = 7. Indeed, denoting by h (H) the (1,0)-step of weight 1 (2), and u=(1,1), d=(1,-1), the five paths of weight 3 are ud, du, hH, Hh, and hhh; they contain 0+0+2+2+3=7 (1,0)-steps at level 0.
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MAPLE
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G:=z*(1+z)/(1+z+z^2)/(1-3*z+z^2): Gser:=series(G, z=0, 32): seq(coeff(Gser, z, n), n=0..28);
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MATHEMATICA
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LinearRecurrence[{2, 1, 2, -1}, {0, 1, 3, 7}, 30] (* Harvey P. Dale, Jan 05 2022 *)
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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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