Concentration properties of semi-vertex transitive graphs and random   bi-coset graphs
  It is well-known that concentrators are sparse graphs of high connectivity, which play a key role in the construction of switching networks; and any semi-vertex transitive graph is isomorphic to a bi-coset graph. In this paper, we prove that random bi-coset graphs are almost always concentrators, and construct some examples of semi-vertex transitive concentrators. 
Xingchao Deng and Kainan Xiang
math.CO