1. Department of Mathematics, Center of Exact Sciences, State University of Londrina, Celso Garcia Cid PR 445 Km 380, Londrina, PR 86051-990, Brazil; PH; +55(43)3371-4226; FAX +55(43)3372-4236;
2. Department of Civil Construction, Polytechnic School, University of São Paulo, Av. Prof. Almeida Prado, Trav. 2, n. 83, São Paulo, SP 05508-070, Brazil; PH +55(11) 3091-5284; FAX +55(11)3091-5715;