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 A027416 Number of unlabeled (and unrooted) trees on n nodes having a centroid. 8
 1, 1, 0, 1, 1, 3, 3, 11, 13, 47, 61, 235, 341, 1301, 1983, 7741, 12650, 48629, 82826, 317955, 564225, 2144505, 3926353, 14828074, 27940136, 104636890, 201837109, 751065460, 1479817181, 5469566585, 10975442036, 40330829030, 82270184950 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,6 COMMENTS Also, number of rooted unlabeled trees on n nodes not having a primary branch. A tree has either a center or a bicenter and either a centroid or a bicentroid. (These terms were introduced by Jordan.) If the number of edges in a longest path in the tree is 2m, then the middle node in the path is the unique center, otherwise the two middle nodes in the path are the unique bicenters. On the other hand, define the weight of a node P to be the greatest number of nodes in any subtree connected to P. Then either there is a unique node of minimal weight, the centroid of the tree, or there is a unique pair of minimal weight nodes, the bicentroids. Let T be a tree with root node R. If R and the edges incident with it are deleted, the resulting rooted trees are called branches. A primary branch (there can be at most one) has i nodes where n/2 <= i <= n-1. REFERENCES F. Harary, Graph Theory, Addison-Wesley, Reading, MA, 1994; pp. 35, 36. LINKS N. J. A. Sloane, Table of n, a(n) for n = 0..200 A. Cayley, On the analytical forms called trees, Amer. J. Math., 4 (1881), 266-268. C. Jordan, Sur les assemblages des lignes, J. Reine angew. Math., 70 (1869), 185-190. A. Meir and J. W. Moon, On the branch-sizes of rooted unlabeled trees, in "Graph Theory and Its Applications", Annals New York Acad. Sci., Vol. 576, 1989, pp. 399-407. E. M. Rains and N. J. A. Sloane, On Cayley's Enumeration of Alkanes (or 4-Valent Trees), J. Integer Sequences, Vol. 2 (1999), Article 99.1.1. [This articles states incorrectly that A000676 and A000677 give the numbers of trees with respectively a centroid and bicentroid.] Peter Steinbach, Field Guide to Simple Graphs, Volume 1, Part 17 (For Volumes 1, 2, 3, 4 of this book see A000088, A008406, A000055, A000664, respectively.) Peter Steinbach, Field Guide to Simple Graphs, Volume 3, Part 12 (For Volumes 1, 2, 3, 4 of this book see A000088, A008406, A000055, A000664, respectively.) FORMULA a(n) = A000055(n) - A102911(n/2) if n is even, else a(n) = A000055(n). a(n) = A000081(n) - A027415(n). - Emeric Deutsch, Nov 21 2004 MAPLE N := 50: Y := [ 1, 1 ]: for n from 3 to N do x*mul( (1-x^i)^(-Y[ i ]), i=1..n-1); series(%, x, n+1); b := coeff(%, x, n); Y := [ op(Y), b ]; od: P:=n->sum(Y[n-i]*Y[i], i=1..floor(n/2)): seq(Y[n]-P(n), n=1..35); # Emeric Deutsch CROSSREFS Cf. A102911 (trees with a bicentroid), A027415 (trees without a primary branch), A000676 (trees with a center), A000677 (trees with a bicenter), A000055 (trees), A000081 (rooted trees). Sequence in context: A045494 A217712 A321678 * A281905 A278835 A163932 Adjacent sequences:  A027413 A027414 A027415 * A027417 A027418 A027419 KEYWORD nonn AUTHOR EXTENSIONS More terms from Emeric Deutsch, Nov 21 2004 Entry revised (with new definition) by N. J. A. Sloane, Feb 26 2007 STATUS approved

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Last modified June 23 17:39 EDT 2021. Contains 345402 sequences. (Running on oeis4.)