S. Mercuri. (2010)cite arxiv:1001.1330
Comment: 91 pages, based on two invited lectures given at the 5th
International School on Field Theory and Gravitation, April 20-24, 2009
Cuiaba city, Brazil.
P. Tuckey. (1993)cite arxiv:gr-qc/9303011
Comment: 5 pages (plus one figure not included, available from author on
request), Plain Tex, no local preprint number (Only change: omitted
"\magnification" command now replaced).