Braided surfaces and seifert ribbons for closed braids

Apositive band in the braid groupB _n is a conjugate of one of the standard generators; a negative band is the inverse of a positive band. Using the geometry of the configuration space, a theory of bands andbraided surfaces is developed. Each representation of a braid as a product of bands yields a handle decomposition of aSeifert ribbon bounded by the corresponding closed braid; and up to isotopy all Seifert ribbons occur in this manner. Thus,band representations provide a convenient calculus for the study of ribbon surfaces. For instance, from a band representation, a Wirtinger presentation of the fundamental group of the complement of the associated Seifert ribbon inD ⁴ can be immediately read off, and we recover a result of T. Yajima (and D. Johnson) that every Wirtinger-presentable group appears as such a fundamental group. In fact, we show that every such group is the fundamental group of a Stein manifold, and so that there are finite homotopy types among the Stein manifolds which cannot (by work of Morgan) be realized as smooth affine algebraic varieties.