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Yang-Mills mass gap at large-N, non-commutative YM theory, topological quantum field theory and hyperfiniteness

We review a number of old and new concepts in quantum gauge theories, some of which are well established but not widely appreciated, some are most recent. Such concepts involve non-commutative gauge theories and their relation to the large-N limit, loop equations and the change to the anti-selfdual variables also known as Nicolai map, topological field theory (TFT) and its relation to localization and Morse-Smale-Floer homology, with an emphasis both on the mathematical aspects and the physical meaning. These concepts, assembled in a new way, enter a line of attack to the problem of the mass gap in large-N SU(N) YM, that is reviewed as well. In the large-N limit of pure SU(N) YM the ambient algebra of Wilson loops is known to be a type II_1 non-hyperfinite factor. Nevertheless, for the mass gap problem at the leading 1/N order, only the subalgebra of local gauge-invariant single-trace operators matters. The connected two-point correlators in this subalgebra must be an infinite sum of propagators of free massive fields, a vast simplification. It is an open problem, determined by the grow of the degeneracy of the spectrum, whether the aforementioned local subalgebra is in fact hyperfinite. For the mass-gap problem, in the search of a hyperfinite subalgebra containing the scalar sector of large-N YM, a major role is played by the existence of a TFT underlying the large-N limit of YM, with twisted boundary conditions on a torus or, what is the same by Morita duality, on a non-commutative torus.