1. Department of Mathematics, University of Chester, Chester, CH1 4BJ, UK
2. CM — UTAD, Department of Mathematics, University of Trás-os-Montes e Alto Douro, Quinta de Prados, 5001-801, Vila Real, Portugal
3. Department of Mathematics, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, Monte da Caparica, 2829-516, Caparica, Portugal