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A109193
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Triangle read by rows: T(n,k) is number of Grand Motzkin paths of length n having k returns to the x-axis (i.e. d or u steps hitting the x-axis). (A Grand Motzkin path is a path in the half-plane x>=0, starting at (0,0), ending at (n,0) and consisting of steps u=(1,1), d=(1,-1) and h=(1,0).).
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1
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1, 1, 1, 2, 1, 6, 1, 14, 4, 1, 30, 20, 1, 64, 68, 8, 1, 140, 196, 56, 1, 318, 524, 248, 16, 1, 750, 1356, 888, 144, 1, 1828, 3476, 2832, 784, 32, 1, 4576, 8932, 8448, 3344, 352, 1, 11700, 23136, 24248, 12368, 2272, 64, 1, 30420, 60528, 68120, 41808, 11232, 832, 1
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,4
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COMMENTS
| Row n contains 1+floor(n/2) terms. Row sums yield the central trinomial coefficients (A002426). T(n,0)=1. sum(k*T(n,k),k=0..floor(n/2))=A109194(n).
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FORMULA
| G.f.=1/(1-z-2tz^2*M), where M=1+zM+z^2*M^2=[1-z-sqrt(1-2z-3z^2)]/(2z^2) is the g.f. of the Motzkin numbers (A001006).
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EXAMPLE
| T(4,2)=4 because we have udud, dudu, uddu and duud, where u=(1,1),d=(1,-1), h=(1,0).
Triangle begins:
1;
1;
1,2;
1,6;
1,14,4;
1,30,20;
1,64,68,8;
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MAPLE
| M:=(1-z-sqrt(1-2*z-3*z^2))/2/z^2: G:=1/(1-z-2*t*z^2*M): Gser:=simplify(series(G, z=0, 17)): P[0]:=1: for n from 1 to 14 do P[n]:=coeff(Gser, z^n) od: for n from 0 to 14 do seq(coeff(t*P[n], t^k), k=1..1+floor(n/2)) od; # yields sequence in triangular form
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CROSSREFS
| Cf. A002426, A001006, A109194.
Sequence in context: A053785 A060173 A059344 * A186283 A173279 A083720
Adjacent sequences: A109190 A109191 A109192 * A109194 A109195 A109196
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KEYWORD
| nonn,tabf
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AUTHOR
| Emeric Deutsch (deutsch(AT)duke.poly.edu), Jun 22 2005
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