[1] Belarusian State Univ, Dept Math & Mech, Minsk 220050, BELARUS
[2] Univ Complutense, Fac Informat, Dept Matemat Aplicada, E-28040 Madrid, Spain
[3] Univ La Laguna, Dept Anal Matemat, San Cristobal la Laguna 38271, Spain
[4] CSIC, INTA, Ctr Astrobiol, Madrid 28850, Spain