Yasuyuki Kachi
Department of Mathematics
University of Kansas
Lawrence, KS 66045-7523
USA
mailto:kachi@math.ku.edukachi@math.ku.edu
Pavlos Tzermias
Department of Mathematics
University of Patras
26500 Rion (Patras)
Greece
mailto:tzermias@math.upatras.grtzermias@math.upatras.gr