A Bethe lattice or Cayley tree (a particular kind of Cayley graph), introduced by Hans Bethe in 1935, is an infinite connected cycle-free graph where each node is connected to z neighbours, where z is called the coordination number. It is a rooted tree, with all other nodes arranged in shells around the root node, also called the origin of the lattice. The number of nodes in the kth shell is given by
In some situations the definition is modified to specify that the root node has z − 1 neighbors.
Due to its distinctive topological structure, the statistical mechanics of lattice models on this graph are often exactly solvable. The solutions are related to the often used Bethe approximation for these systems.
Relation to Cayley graphs
A presentation of a group G by n generators corresponds to a surjective map from the free group on n generators to the group G, and at the level of Cayley graphs to a map from the Cayley tree to the Cayley graph. This can also be interpreted (in algebraic topology) as the universal cover of the Cayley graph, which is not in general simply connected.
The distinction between a Bethe lattice and a Cayley tree is that the former is infinite, while the latter is finite, so that a Bethe lattice has no surface and no root, whereas in Cayley trees the surface is highly non-negligible.