Cayley's formula ($n^{n-2}$)

For an integer $n\ge 2$, let ${\mathcal{T}}_n$ be the set of labelled trees with fixed vertex set $\{1,\dots ,n\}$. Each tree is a finite simple graph, with no distinguished root of a tree, and two trees are different exactly when their sets of edges differ. ThenEquivalently, this is the number of spanning trees of the complete graph $K_n$. The labelled combinatorial structure includes the fixed labels: drawings, positions, and the order in which edges are listed do not create extra trees. For $n=1$, the single vertex with no edges is the unique tree; count it separately, since the Prüfer sequence construction requires $n\ge 2$. For $n=2$, the unique tree has edge $\{1,2\}$ and corresponds to the empty Prüfer sequence. ^[ref]

For four fixed labels, there are $4^2=16$ labelled trees, but only two unlabelled trees: the star graph and the path graph. There are four star graphs, one for each choice of centre. There are $4!/2=12$ path graphs: arrange the labels along a graph path, then identify each ordering with its reversal, which describes the same set of edges. Thus $4+12=16$. For instance, star graphs centred at $1$ and at $2$ have different sets of edges even though they have the same shape.

The striking point is that connectedness and the absence of a graph cycle are global restrictions on edges, yet the answer is a power with no correction factor. The Prüfer sequence supplies a bijection that turns those restrictions into $n-2$ freely chosen labels. The rule of product then gives $n^{n-2}$ immediately. The same representation makes vertex degrees easy to count and supports uniform labelled tree generation for simulations and algorithm experiments.

Arthur Cayley's 1889 paper "A theorem on trees", published in the Quarterly Journal of Pure and Applied Mathematics, volume 23, pages 376–378, treats the counting problem and its refinement by vertex degrees. It was reprinted as paper 895 in volume XIII of his Collected Mathematical Papers, pages 26–28. This publication is the historical context for the name Cayley's formula; the name alone is not a claim of priority. ^{[ref] [ref]}