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A143364
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Triangle read by rows: T(n,k) is the number of {0-1-2}-trees with n edges and k protected vertices (0<=k<=n-1). A {0-1-2}-tree is an ordered tree in which the outdegree of every vertex is 0, 1, or 2. A protected vertex in an ordered tree is a vertex at least 2 edges away from its leaf descendants.
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1
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1, 1, 1, 2, 1, 1, 2, 5, 1, 1, 4, 6, 9, 1, 1, 4, 19, 12, 14, 1, 1, 8, 24, 53, 20, 20, 1, 1, 8, 62, 78, 116, 30, 27, 1, 1, 16, 80, 250, 190, 220, 42, 35, 1, 1, 16, 184, 382, 735, 390, 379, 56, 44, 1, 1, 32, 240, 1020, 1270, 1785, 714, 609, 72, 54, 1, 1, 32, 512, 1580, 3900, 3390, 3808
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OFFSET
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1,4
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COMMENTS
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Row sums are the Motzkin numbers (A001006).
T(n,0) is the sequence 1,1,2,2,4,4,8,8,16,16,... (A016116).
Sum(k*T(n,k),k=0..n-1) = A143335(n).
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LINKS
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FORMULA
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G.f.: g, where g=g(t,z) satisfies tz^2*g^2-(1-tz-2z^2)g+z(1+z)=0.
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EXAMPLE
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Triangle starts:
1;
1,1;
2,1,1;
2,5,1,1;
4,6,9,1,1;
4,19,12,14,1,1;
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MAPLE
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g:=((1-t*z-2*z^2-sqrt((1-t*z)^2-4*z^2*(1-z^2+t*z^2)))*1/2)/(t*z^2): gser:= simplify(series(g, z=0, 16)): for n to 12 do P[n]:=sort(coeff(gser, z, n)) end do: for n to 12 do seq(coeff(P[n], t, j), j=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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