1. Department of Mathematics, California State University, Northridge, 18111 Nordho Street, Northridge, CA 91330, United States of America
2. Department of Mathematics, San Francisco State University, 1600 Holloway Ave, San Francisco, CA 94132, United States of America
3. Department of Mathematics, California State University at Fullerton, 800 N. State College Blvd., Fullerton, CA, 92834-6850, United States of America