Countable connected Hausdorff and Urysohn bunches of arcs in the plane
by
Piotr Minc
Auburn University

We answer a question by Krasinkiewicz, Re\'nska and Sobolewski by constructing countable connected Hausdorff and Urysohn spaces as quotient spaces of bunches of arcs in the plane. We also consider a generalization of graphs by allowing vertices to be continua and replacing edges by not necessarily connected sets. We require only that two "vertices" be in the same quasi-component of the "edge" that contains them. We observe that if a graph G cannot be embedded in the plane, then any generalized graph modeled on G is not embeddable in the plane. As a corollary we obtain not planar bunches of arcs with their natural quotients Hausdorff or Urysohn. This answers another question by Krasinkiewicz, Re\'nska and Sobolewski.

Date received: February 23, 2005