I. Benjamini et O. Schramm, EVERY GRAPH WITH A POSITIVE CHEEGER CONSTANT CONTAINS A TREE WITH A POSITIVE CHEEGER CONSTANT, Geometric and functional analysis, 7(3), 1997, pp. 403-419
It is shown that every (infinite) graph with a positive Cheeger consta
nt contains a tree with a positive Cheeger constant. Moreover, for eve
ry nonnegative integer k there is a unique connected graph T(k) that h
as Cheeger constant k, but removing any edge from it reduces the Cheeg
er constant. This minimal graph, T(k), is a tree, and every graph G wi
th Cheeger constant h(G) greater than or equal to k has a spanning for
est in which each component is isomorphic to T(k).