Affiliation: | 1. Department of Mathematics, Kansas State University, Mahattan, KS 66506, United States of America;2. Department of Mathematics, Landzhou University, Lanzhou, China;1. Departamento de Matemáticas, Universidad de La Frontera, Casilla 54-D, Temuco, Chile;2. Departamento de Matemáticas, Universidad del Valle, Ciudad Universitaria Meléndez, Calle 13 # 100-00, Código Postal 760032, Cali, Colombia;1. Instituto Federal Sul-rio-grandense, Brazil;2. Universidade Federal do Rio Grande do Sul, Brazil |