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1, 3, 10, 33, 110, 366, 1220, 4065, 13550, 45162, 150540, 501786, 1672620, 5575356, 18584520, 61948257, 206494190, 688313490, 2294378300, 7647926046, 25493086820, 84976950468, 283256501560, 944188318938, 3147294396460
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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0,2
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COMMENTS
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Hankel transform is A000012=[1,1,1,1,1,1,1,...].
a(n) is the number of Motzkin paths of length n in which the (1,0)-steps at level 0 come in 3 colors and there are no (1,0)-steps at a higher level. Example: a(3)=33 because, denoting U=(1,1), H=(1,0), and D=(1,-1), we have 3^3 = 27 paths of shape HHH, 3 paths of shape HUD, and 3 paths of shape UDH. - Emeric Deutsch, May 02 2011
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LINKS
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FORMULA
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G.f.: 1/(1-3x-x^2/(1-x^2/(1-x^2/(1-x^2/(1-... (continued fraction). - Paul Barry, Mar 10 2009
Conjecture: 3*(n+1)*a(n) +10*(-n-1)*a(n-1) +12*(-n+2)*a(n-2) +40*(n-2)*a(n-3)=0. - R. J. Mathar, Nov 26 2012
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MAPLE
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MATHEMATICA
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With[{s = Partition[#, 2, 1] &@ Array[Sum[Binomial[#, Floor[k/2]]*3^(# - k), {k, 0, #}] &, 26, 0]}, Map[#2/2 - #1 & @@ # &, s]] (* Michael De Vlieger, Dec 15 2019 *)
CoefficientList[Series[2/(1-6*x+Sqrt[1-4*x^2]), {x, 0, 30}], x] (* G. C. Greubel, Jan 29 2020 *)
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PROG
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(PARI) my(x='x+O('x^30)); Vec( 2/(1 - 6*x + sqrt(1-4*x^2)) ) \\ G. C. Greubel, Jan 29 2020
(Magma) R<x>:=PowerSeriesRing(Rationals(), 30); Coefficients(R!( 2/(1 - 6*x + Sqrt(1-4*x^2)) )); // G. C. Greubel, Jan 29 2020
(Sage)
P.<x> = PowerSeriesRing(ZZ, prec)
return P( 2/(1 - 6*x + sqrt(1-4*x^2)) ).list()
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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