Affiliation: | aDepartment of Mathematics, East Tennessee State University, Johnson City, TN 37614-0002, USA;bDepartment of Mathematics, University of Johannesburg, Auckland Park 2006, South Africa;cDepartment of Mathematics, University of Tennessee at Chattanooga, Chattanooga, TN 37403, USA;dDepartment of Computer Science, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, UK |