Wedderburn–Etherington number

The Wedderburn–Etherington numbers are a sequence of integers that count certain types of binary trees, specifically the number of distinct full binary trees (or proper binary trees) with a given number of internal nodes. A full binary tree is a tree in which every internal node has exactly two children. The \( n \)-th Wedderburn–Etherington number counts the number of full binary trees with \( n \) internal nodes.