Motzkin numbers. Number of rooted planar unlabeled unary-binary trees with N nodes. Use this tag when calculating Motzkin numbers, e.g. 1, 1, 2, 4, 9, 21, 51, 127, or with enumerating unary-binary trees, or binary expression trees, or related combinatorics . Note: Don't confuse binary expression trees with strictly binary trees used with expressions, binary expression trees have unary edges.
