[1] Univ Regensburg, Inst Theoret Phys, D-93040 Regensburg, Germany
[2] Univ Complutense Madrid, Dept Fis Mat, E-28040 Madrid, Spain
[3] UPM, Dpto Matemat Aplicada & Estadist, EUIT Aeronaut, Madrid 28040, Spain
[4] Univ Warwick, Dept Phys, Coventry CV4 7AL, W Midlands, England