[1] Univ Beira Interior, CMA, Rua Marques dAvila & Bolama, P-6201001 Covilha, Portugal
[2] Univ Beira Interior, Dept Matemat, Rua Marques dAvila & Bolama, P-6201001 Covilha, Portugal
[3] Univ Coimbra, Dept Math, Apartado 3008, P-3001501 Coimbra, Portugal