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 A120988 Triangle read by rows: T(n,k) is the number of binary trees with n edges and such that the first leaf in the preorder traversal is at level k (1<=k<=n). A binary tree is a rooted tree in which each vertex has at most two children and each child of a vertex is designated as its left or right child. 1
 2, 1, 4, 2, 4, 8, 5, 9, 12, 16, 14, 24, 30, 32, 32, 42, 70, 85, 88, 80, 64, 132, 216, 258, 264, 240, 192, 128, 429, 693, 819, 833, 760, 624, 448, 256, 1430, 2288, 2684, 2720, 2490, 2080, 1568, 1024, 512, 4862, 7722, 9009, 9108, 8361, 7068, 5488, 3840, 2304, 1024 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Row sums are the Catalan numbers (A000108). T(n,1)=A000108(n-1) for n>=2 (the Catalan numbers). T(n,n)=2^n. Sum(k*T(n,k),k=1..n)=A120989(n). LINKS FORMULA T(n,k)=Sum(j*binomial(k,j)*binomial(2n-2k+j,n-k)/(2n-2k+j), j=0..k). G.f.=1/[1-tz(1+C)], where C=[1-sqrt(1-4z)]/(2z) is the Catalan function. EXAMPLE T(2,1)=1 because we have the tree /\. Triangle starts: 2; 1;4; 2,4,8; 5,9,12,16; 14,24,30,32,32; MAPLE T:=proc(n, k) if k

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