Valdés-Bango F ; García Alonso F.J ; Rodríguez-Rodríguez G... Work supported by Spanish MEC under Grants FIS2008-06249 and Consolider CSD2007-00010 and by CAM under Grant S2009/MAT-1726. We acknowledge helpful discussions with Professor T. P. Russell and Dr. R. Morales..
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2020