Infinite homogeneous bipartite graphs with unequal sides

Abstract

We call a bipartite graph {\it homogeneous} if every finite partial automorphism which respects left and right can be extended to a total automorphism. A $(\kappa,{\lambda} )$ bipartite graph is a bipartite graph with left side of size $\kappa$ and right side of size ${\lambda}$. We show, using a theorem of Hrushovski on finite graphs, that there is a homogeneous $({\aleph_0},2^{\aleph_0} )$ bipartite graph of girth 4 (thus answering negatively a question by Kupitz and Perles), and that depending on the underlying set theory all homogeneous $({\aleph_0},\aleph_1)$ bipartite graphs may be isomorphic, or there may be $2^{\aleph_1}$ many isomorphism types of $(\aleph_0,\aleph_1)$ homogeneous graphs.