Fivebranes are non-perturbative objects in string theory that generalize
two-dimensional conformal field theory and relate such diverse subjects as
moduli spaces of vector bundles on surfaces, automorphic forms, elliptic
genera, the geometry of Calabi-Yau threefolds, and generalized Kac-Moody
algebras.Comment: 10 pages, 2 figures, Lecture at ICM'9