2005/08/09 by Michael R. Douglas, Douglas, Michael R.
Mathematics · Physics and Astronomy · #FOS: Physical sciences #Mathematical Physics (math-ph) #math-ph #math.MP
paper · pdf · doi:10.48550/arxiv.math-ph/0508019
16 pages, Latex
arxiv created 2005/08/09 · arxiv updated 2009/12/01
This is a submission to the Encyclopedia of Mathematical Physics (Elsevier, 2006) and conforms to its referencing guidelines.