Concept

# Wedderburn–Etherington number

Summary
The Wedderburn–Etherington numbers are an integer sequence named for Ivor Malcolm Haddon Etherington and Joseph Wedderburn that can be used to count certain kinds of binary trees. The first few numbers in the sequence are 0, 1, 1, 1, 2, 3, 6, 11, 23, 46, 98, 207, 451, 983, 2179, 4850, 10905, 24631, 56011, ... () These numbers can be used to solve several problems in combinatorial enumeration. The nth number in the sequence (starting with the number 0 for n = 0) counts The number of unordered rooted trees with n leaves in which all nodes including the root have either zero or exactly two children. These trees have been called Otter trees, after the work of Richard Otter on their combinatorial enumeration. They can also be interpreted as unlabeled and unranked dendrograms with the given number of leaves. The number of unordered rooted trees with n nodes in which the root has degree zero or one and all other nodes have at most two children. Trees in which the root has at most one child are called planted trees, and the additional condition that the other nodes have at most two children defines the weakly binary trees. In chemical graph theory, these trees can be interpreted as isomers of polyenes with a designated leaf atom chosen as the root. The number of different ways of organizing a single-elimination tournament for n players (with the player names left blank, prior to seeding players into the tournament). The pairings of such a tournament may be described by an Otter tree. The number of different results that could be generated by different ways of grouping the expression for a binary multiplication operation that is assumed to be commutative but neither associative nor idempotent. For instance can be grouped into binary multiplications in three ways, as , , or . This was the interpretation originally considered by both Etherington and Wedderburn. An Otter tree can be interpreted as a grouped expression in which each leaf node corresponds to one of the copies of and each non-leaf node corresponds to a multiplication operation.
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