1973, ISBN 978-0-521-09906-6 . The original work is Ehlers, J., Geren, P., Sachs, R.K.: Isotropic solutions of Einstein-Liouville equations. J. Math. Phys
Masters, Karen L.; Merino-Viteri, Andrés; Puschendorf, Robert; Ron, Santiago R.; Sánchez-Azofeifa, G. Arturo; Still, Christopher J.; Young, Bruce E. Widespread