O. Post. (2007)cite arxiv:0712.1507Comment: 24 pages, extended version of a lecture held at the EPF Lausanne, some references added and typos corrected.
H. Cao, и X. Zhu. (2006)cite arxiv:math/0612069Comment: This is a revised version of the article by the same authors that originally appeared in Asian J. Math., 10(2) (2006), 165--492.