The complete list of all trees on 2,3,4 labeled vertices: ${\displaystyle 2^{2-2}=1}$ tree with 2 vertices, ${\displaystyle 3^{3-2}=3}$ trees with 3 vertices and ${\displaystyle 4^{4-2}=16}$ trees with 4 vertices.

In mathematics, Cayley's formula is a result in graph theory named after Arthur Cayley. It states that for every positive integer ${\displaystyle n}$, the number of trees on ${\displaystyle n}$ labeled vertices is ${\displaystyle n^{n-2}}$.

The formula equivalently counts the number of spanning trees of a complete graph with labeled vertices (sequence A000272 in the OEIS).

Proof

Many proofs of Cayley's tree formula are known.^[1] One classical proof of the formula uses Kirchhoff's matrix tree theorem, a formula for the number of spanning trees in an arbitrary graph involving the determinant of a matrix. Prüfer sequences yield a bijective proof of Cayley's formula. Another bijective proof, by André Joyal, finds a one-to-one transformation between n-node trees with two distinguished nodes and maximal directed pseudoforests. A proof by double counting due to Jim Pitman counts in two different ways the number of different sequences of directed edges that can be added to an empty graph on n vertices to form from it a rooted tree; see Double counting (proof technique) § Counting trees.

History

The formula was first discovered by Carl Wilhelm Borchardt in 1860, and proved via a determinant.^[2] In a short 1889 note, Cayley extended the formula in several directions, by taking into account the degrees of the vertices.^[3] Although he referred to Borchardt's original paper, the name "Cayley's formula" became standard in the field.

Other properties

Cayley's formula immediately gives the number of labelled rooted forests on n vertices, namely (n + 1)^{n − 1}. Each labelled rooted forest can be turned into a labelled tree with one extra vertex, by adding a vertex with label n + 1 and connecting it to all roots of the trees in the forest.

There is a close connection with rooted forests and parking functions, since the number of parking functions on n cars is also (n + 1)^{n − 1}. A bijection between rooted forests and parking functions was given by M. P. Schützenberger in 1968.^[4]

References