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A191523
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Triangle read by rows: T(n,k) is the number of left factors of Dyck paths of length n and having k double rises, i.e., two consecutive (1,1)-steps (n>=1, 0<=k<=n-1).
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1
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1, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 3, 4, 1, 1, 1, 6, 6, 5, 1, 1, 1, 6, 12, 8, 6, 1, 1, 1, 10, 20, 20, 10, 7, 1, 1, 1, 10, 30, 35, 28, 12, 8, 1, 1, 1, 15, 50, 70, 54, 37, 14, 9, 1, 1, 1, 15, 65, 115, 116, 75, 47, 16, 10, 1, 1, 1, 21, 105, 210, 224, 175, 99, 58, 18, 11, 1, 1, 1, 21, 126, 315, 420, 357, 246, 126, 70, 20, 12, 1, 1
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OFFSET
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1,8
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COMMENTS
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Row n contains n entries.
Sum of entries in row n is binomial(n, floor(n/2)) = A001405(n).
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LINKS
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FORMULA
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G.f.: G(t,z)=(z+r+r*z)/(1-t*z*(1+r)) where r=r(t,z) is a solution of z^2*(1+r)*(1+t*r) (the Narayana function with argument z^2).
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EXAMPLE
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T(5,2)=4 because we have UD(U[U)U], (UU)D(UU), (U[U)U]DD, and (U[U)U]DU, where U=(1,1) and D=(1,-1) (the double rises are shown between parentheses).
Triangle starts:
1;
1, 1;
1, 1, 1;
1, 3, 1, 1;
1, 3, 4, 1, 1;
1, 6, 6, 5, 1, 1;
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MAPLE
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eqr := R = z^2*(1+R)*(1+t*R): r := RootOf(eqr, R): G := (z+r+r*z)/(1-t*z*(1+r)): Gser := simplify(series(G, z = 0, 17)): for n to 13 do P[n] := sort(coeff(Gser, z, n)) end do: for n to 13 do seq(coeff(P[n], t, k), k = 0 .. n-1) end do; # yields sequence in triangular form
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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