Localización:Relativity and gravitation in general: proceedings of the Spanish Relativity Meeting in honour of the 65th birthday of Lluís Bel : Salamanca, Spain, 22-25 September 1998 / Jesús Martín Martín (ed. lit.) , Eduardo Ruiz Carrero (ed. lit.) , Fernando Atrio Barandela (ed. lit.) , Alfred Molina i Compte (ed. lit.) , 1999, ISBN 9810239327, págs. 211-214