**Conjecture**If is a highly arc transitive digraph with two ends, then every tile of is a disjoint union of complete bipartite graphs.

It follows from a theorem of Dunwoody [D] that every vertex transitive graph with two ends has a system of imprimitivity with finite blocks so that the cyclic order is preserved by the automorphism group (of ). If is edge-transitive, then every edge of must have its ends in two consecutive blocks, so in this case is an edge-disjoint union of the (isomorphic) bipartite graphs for - which we shall call *tiles*. Note that the tiles are edge-transitive.

This gives us a good description of edge-transitive graphs with two ends; each is made up by gluing together copies of a tile in a linear order. If is a 2-arc transitive digraph with two ends, then all edges in each tile must be oriented consistently, so by possibly reordering, we may assume that every edge in is oriented from to . The above conjecture asserts that under the added symmetry condition of high arc transitivity, each tile has a simple structure - namely it is a union of (consistently oriented) complete bipartite graphs.

It is easy to construct a highly arc transitive two ended graph by simply using the complete bipartite graph (with all edges oriented consistently) as a tile. Mckay and Praeger found the following pretty construction of a highly arc transitive digraph with tiles isomorphic to a disjoint union of complete bipartite graphs: Let be a finite set, let be a positive integer, and define to be the digraph with vertex set and an edge from to if , , and . A generalized (twisted) version of this construction was introduced by Cameron, Praeger, and Wormald [CPW], but again, every tile in this construction is a disjoint unions of bipartite graphs, and it looks hard to do anything else.

## Bibliography

*[CPW] P. J. Cameron, C. E. Praeger, and N. C. Wormald, Infinite highly arc transitive digraphs and universal covering digraphs. Combinatorica 13 (1993), no. 4, 377--396. MathSciNet.

[D] M. J. Dunwoody, Cutting up graphs. Combinatorica 2 (1982), no. 1, 15--23. MathSciNet

* indicates original appearance(s) of problem.