We have significantly altered and restructured the text of the previous version, describing the latest results from a new point of view. New figures have been added. The number of references has increased from 271 to 350.
http://www.livingreviews.org/lrr-2003-2
http://www.livingreviews.org/lrr-2011-1 |
Living Rev. Relativity 14, (2011), 1
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