[1] Univ Vigo, Dept Matemat Aplicada 2, EI Telecomunicac, Vigo 36310, Spain
[2] Univ Santiago Compostela, Fac Matemat, Dept Matemat Aplicada, Santiago 15782, Spain
[3] Univ Santiago Compostela, Dept Matemat Aplicada, EPS, Lugo 27002, Spain
[4] CITMAGA, Santiago De Compostela 15782, Spain