1. Department of Mathematics and Statistics, Loyola University Chicago, Chicago, IL, USA. E-mail: mbocea@luc.edu
2. Department of Mathematics, University of Craiova, Craiova, Romania. E-mail: mmihailes@yahoo.com
3. Department of Mathematics, University of Texas at Austin, Austin, TX, USA. E-mail: mayte@math.utexas.edu
4. Departamento de Análisis Matemático, Universidad de Alicante, Alicante, Spain. E-mail: julio.rossi@ua.es