1. Centro de Investigación en Matemáticas, Universidad Autónoma del Estado de Hidalgo, Pachuca, Hidalgo, Mexico
2. Department of Mathematics, Bowdoin College, 8600 College Station, Brunswick, ME 04011-8486, USA
3. Department of Mathematics, Clarkson University, 8 Clarkson Avenue, Potsdam, NY 13699-5815, USA
4. Division of Mathematics and Computer Science, Clarkson University, 8 Clarkson Avenue, Potsdam, NY 13699-5815, USA
5. Department of Mathematics, Taylor University, 236 West Reade Avenue, Upland, IN 46989-1001, USA
6. Department of Computer Science, Clarkson University, 8 Clarkson Avenue, Potsdam, NY 13699-5815, USA