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A114712
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Triangle read by rows: T(n,k) = number of peakless Motzkin paths of length n and having k ascents (0<=k<=floor(n/3)).
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1
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1, 1, 1, 1, 1, 1, 3, 1, 7, 1, 14, 2, 1, 26, 10, 1, 46, 35, 1, 79, 100, 5, 1, 133, 254, 35, 1, 221, 595, 161, 1, 364, 1316, 588, 14, 1, 596, 2788, 1862, 126, 1, 972, 5714, 5334, 714, 1, 1581, 11408, 14190, 3150, 42, 1, 2567, 22300, 35652, 11850, 462, 1, 4163, 42842
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OFFSET
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0,7
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COMMENTS
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A Motzkin path of length n is a lattice path from (0,0) to (n,0) consisting of U=(1,1), D=(1,-1) and H=(1,0) steps and never going below the x-axis. An ascent in a Motzkin path is a maximal sequence of consecutive U steps.
Row n contains 1+floor(n/3) terms.
Row sums yield the RNA secondary structure numbers (A004148).
T(3n,n) = A000108(n) (the Catalan numbers).
Sum(k*T(n,k),k=0..floor(n/3)) = A114713(n-3) (n>=3).
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LINKS
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FORMULA
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G.f.: G=G(t, z) satisfies G=1+zG+z^2*(tzG+G-1-zG)G.
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EXAMPLE
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T(6,2)=2 because we have (U)HD(U)HD and (U)H(U)HDD (the ascents are shown between parentheses).
Triangle begins:
1;
1;
1;
1,1;
1,3;
1,7;
1,14,2;
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MAPLE
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G:=(1+z^2-z-sqrt(1-z^2-2*z+z^4+2*z^3-4*z^3*t))/2/z^2/(z*t+1-z): Gser:=simplify(series(G, z=0, 22)): P[0]:=1: for n from 1 to 18 do P[n]:=coeff(Gser, z^n) od: for n from 0 to 18 do seq(coeff(t*P[n], t^j), j=1..1+floor(n/3)) od; # yields sequence in triangular form
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CROSSREFS
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KEYWORD
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nonn,tabf
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AUTHOR
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STATUS
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approved
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