Equivariant brst quantization and reducible symmetries

Working from first principles, quantization of a class of symmetric Hamiltonian systems whose constraint algebras are not closed is carried out by constructing first the appropriate reduced phase space and then the brst cohomology. The brst operator constructed is equivariant with respect to a subgroup H of the symmetry group G of the system. Using algebraic techniques from equivariant de Rham theory, the quantization is shown to correspond to the bv quantization of a class of systems with reducible symmetry. As an example of the methods developed, a topological model is described whose brst quantization relates to the equivariant cohomology of a manifold under a circle action. In an appendix the issue of non-unique constraint functions and algebras is clarified.