1. Dept de Matematica, Univ de Beira Interior, Covilha, 6201-001, Portugal
2. Dept de Matematica Aplicada, Univ Complutenese, Madrid, 28040, Spain
3. Dept de Matematicas, Univ de Oviedo, Oviedo, 33007, Spain
4. Dept of Mech, Mat, and Aerospace Eng, Col of Eng, Univ of Central Florida, Orlando, FL 32816-2450