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Search: id:A102004
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| A102004 |
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Triangle read by rows: T(n,k) is the number of ordered trees with n edges and having k branches of even length (n>=0, 0<=k<=floor(n/2)). |
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+0
2
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| 1, 1, 1, 1, 3, 2, 6, 7, 1, 16, 20, 6, 40, 64, 26, 2, 109, 196, 108, 16, 297, 619, 414, 96, 4, 836, 1940, 1557, 484, 45, 2377, 6142, 5690, 2247, 331, 9, 6869, 19454, 20535, 9792, 2010, 126, 20042, 61893, 73123, 40997, 10820, 1116, 21, 59071, 197280, 258220
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OFFSET
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0,5
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COMMENT
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Row n has 1+floor(n/2) terms. Row sums are the Catalan numbers (A000108).T(2n,n)=A001006(n-1) for n>=1 (the Motzkin numbers). T(2n+1,n)=A005717(n+1) for n>=0.
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REFERENCES
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E. Deutsch, Ordered trees with prescribed root degrees, node degrees,and branch lengths, Discrete Math., 282, 2004, 89-94.
J. Riordan, Enumeration of plane trees by branches and endpoints, J.Combin. Theory Ser. A 19, 1975, 214-222.
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FORMULA
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G.f.=G=G(t, z) satisfies z(1+tz)G^2-(1+z-z^2+tz^2)G+1+z-z^2+tz^2=0
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EXAMPLE
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T(3,0)=3 because we have: (i) tree with 3 edges hanging from the root, (ii) tree with one edge hanging from the root, at the end of which 2 edges are hanging, and (iii) tree with a path of length 3 hanging from the root.
Triangle starts:
1;
1;1,1;
3,2;
6,7,1;
16,20,6;
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MAPLE
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G:=1/2/(t*z^2+z)*(-z^2+z+1+t*z^2-sqrt(-5*z^2-6*t*z^3-2*z+2*z^3-3*t^2*z^4-2*t*z^2+2*t*z^4+1+z^4)): Gserz:=simplify(series(G, z=0, 16)): P[0]:=1: for n from 1 to 14 do P[n]:=sort(expand(coeff(Gserz, z^n))) od:for n from 0 to 14 do seq(coeff(t*P[n], t^k), k=1..1+floor(n/2)) od;
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CROSSREFS
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Cf. A000108, A001006, A005717, A102003.
Sequence in context: A014686 A053090 A087237 this_sequence A125764 A023897 A100527
Adjacent sequences: A102001 A102002 A102003 this_sequence A102005 A102006 A102007
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KEYWORD
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nonn,tabf
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AUTHOR
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Emeric Deutsch (deutsch(AT)duke.poly.edu), Dec 25 2004
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