Apr. 2     N.CALIF SYMPLECTIC GEOMETRY SEMINAR  [A.Weinstein et al]
           2:10,  891 Evans
            Tamas Hausel, UCB
            Cohomology ring of hyperkahler quotients
First I discuss the present knowledge about the cohomology ring of a
symplectic quotient, mentioning two key results: the surjectity of the
Kirwan map and Martin's formula relating the ring to the cohomology ring
of the associated quotient by a maximal torus. Then I will discuss the
few scattered results about the cohomology of hyperkahler quotients in 
special cases like Nakajima's results and Lusztig's conjecture about the
cohomology of quiver varieties and in a special case the calculation by
Konno of the cohomology ring of the hyperkahler polygon spaces.  I will
finish by describing a theory which attempts a thorough understanding 
of the cohomology ring structure of circle-compact hyperkahler quotients.