Noncrossing Trees and Noncrossing Graphs

William Y. C. Chen and Sherry H. F. Yan

  Abstract:  We give a parity reversing involution on noncrossing trees that leads to a combinatorial interpretation of a formula on noncrossing trees and symmetric ternary trees in answer to a problem proposed by Hough. We use the representation of Panholzer and Prodinger for noncrossing trees and find a correspondence between a class of noncrossing trees, called proper noncrossing trees, and the set of symmetric ternary trees. The second result of this paper is a parity reversing involution on connected noncrossing graphs which leads to a relation between the number of noncrossing trees with n edges and k descents and the number of connected noncrossing graphs with n + 1 vertices and m edges.

  AMS Classification:  05A05, 05C30


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