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 A120982 Triangle read by rows: T(n,k) is the number of ternary trees with n edges and having k vertices of outdegree 2 (n >= 0, k >= 0). 4
 1, 3, 9, 3, 28, 27, 93, 162, 18, 333, 825, 270, 1272, 3915, 2430, 135, 5085, 18144, 17199, 2835, 20925, 84000, 106596, 34020, 1134, 87735, 391554, 612360, 308448, 30618, 372879, 1838295, 3369600, 2364390, 459270, 10206, 1602450, 8674380 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS A ternary tree is a rooted tree in which each vertex has at most three children and each child of a vertex is designated as its left or middle or right child. LINKS FORMULA T(n,k) = (1/(n+1))*binomial(n+1,k)*Sum_{j=0..floor(n/2)-k} 3^(n-k-3j)*binomial(n+1-k, k+1+2j)*binomial(n-2k-2j, j). G.f.: G = G(t,z) satisfies G = 1 + 3zG + 3tz^2*G^2 + z^3*G^3. Row n has 1+floor(n/2) terms. Row sums yield A001764. T(n,0) = A120985(n). Sum_{k>=1} k*T(n,k) = 3*binomial(3n,n-2) = 3*A003408(n-2). EXAMPLE T(2,1)=3 because we have (Q,L,M), (Q,L,R) and (Q,M,R), where Q denotes the root and L (M,R) denotes a left (middle, right) child of Q. Triangle starts:     1;     3;     9,   3;    28,  27;    93, 162,  18;   333, 825, 270; MAPLE T:=(n, k)->(1/(n+1))*binomial(n+1, k)*sum(3^(n-k-3*q)*binomial(n+1-k, k+1+2*q)*binomial(n-2*k-2*q, q), q=0..n/2-k):for n from 0 to 12 do seq(T(n, k), k=0..floor(n/2)) od; # yields sequence in triangular form CROSSREFS Cf. A001764, A003408, A120429, A120981, A120983, A120985. Sequence in context: A339882 A120429 A101431 * A293634 A125143 A200012 Adjacent sequences:  A120979 A120980 A120981 * A120983 A120984 A120985 KEYWORD nonn,tabf AUTHOR Emeric Deutsch, Jul 21 2006 STATUS approved

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Last modified April 15 01:24 EDT 2021. Contains 342974 sequences. (Running on oeis4.)