International Journal of Mathematics

In this paper, we extend the local gradient estimates established in [8, 10] to the conformal quotient equations. An existence of solutions of the conformal quotient equations follows from the local gradient estimates and the paper [14] of Gursky and Viaclovsky.

In this paper, we continue the investigation of coherent systems of type (n,d,k) on the projective line which are stable with respect to some value of a parameter α. We work mainly with k < n and obtain existence results for arbitrary k in certain cases, together with complete results for k = 3. Our methods involve the use of the "flips" which occur at critical values of the parameter.

Twisted homology groups attached to a Selberg type integral are studied under some resonance condition. We study the kernel of the map from the homology of compact cycles to the homology of locally finite cycles. As a result, the dimension of the image, the space of the regularizable cycles, is expressed as a special value of the generalized hypergeometric series ₃F₂.

Let C be an algebraic curve of genus g ≥ 2. A coherent system on C consists of a pair (E,V), where E is an algebraic vector bundle over C of rank n and degree d and V is a subspace of dimension k of the space of sections of E. The stability of the coherent system depends on a parameter α. We study the geometry of the moduli space of coherent systems for different values of α when k ≤ n and the variation of the moduli spaces when we vary α. As a consequence, for sufficiently large α, we compute the Picard groups and the first and second homotopy groups of the moduli spaces of coherent systems in almost all cases, describe the moduli space for the case k = n - 1 explicitly, and give the Poincaré polynomials for the case k = n - 2. In an appendix, we describe the geometry of the "flips" which take place at critical values of α in the simplest case, and include a proof of the existence of universal families of coherent systems when GCD(n,d,k) = 1.

In this paper, we describe the commutant of an arbitrary subalgebra A of the algebra of functions on a set X in a crossed product of A with the integers, where the latter act on A by a composition automorphism defined via a bijection of X. The resulting conditions which are necessary and sufficient for A to be maximal abelian in the crossed product are subsequently applied to situations where these conditions can be shown to be equivalent to a condition in topological dynamics. As a further step, using the Gelfand transform, we obtain for a commutative completely regular semi-simple Banach algebra a topological dynamical condition on its character space which is equivalent to the algebra being maximal abelian in a crossed product with the integers.