\ \ In 1939 Rademacher derived a conditionally convergent series expression for the modular j-invariant, and used this expression---the first Rademacher sum---to verify its modular invariance. We may attach Rademacher sums to other discrete groups of isometries of the hyperbolic plane, and we may ask how the automorphy of the resulting functions reflects the geometry of the group in question.
\\
\ \ In the case of a group that defines a genus zero quotient of the hyperbolic plane the relationship is particularly striking. On the other hand, of the common features of the groups that arise in monstrous moonshine, the genus zero property is perhaps the most elusive. We will illustrate how Rademacher sums elucidate this phenomena by using them to formulate a characterization of the discrete groups of monstrous moonshine.
\\
\ \ A physical interpretation of the Rademacher sums comes into view when we consider black holes in the context of three dimensional quantum gravity. This observation, together with the application of Rademacher sums to moonshine, amounts to a new connection between moonshine, number theory and physics, and furnishes applications in all three fields.
There is a conjectural relationship due to Yau-Tian-Donaldson between stability of projective manifolds and the existence of canonical Kahler metrics (e.g. Kahler-Einstein metrics). Embedding the projective manifold in a large projective space gives, on one hand, a Geometric Invariant Theory stability problem (by changing coordinates on the projective space) and, on the other, a notion of balanced metric which can be used to approximate the canonical Kahler metric in question. I shall discuss joint work with Richard Thomas that extends this framework to orbifolds with cyclic quotient singularities using embeddings in weighted projective space, and examples that show how several obstructions to constant scalar curvature orbifold metrics can be interpreted in terms of stability.
The 4D rotating black hole described by the Kerr geometry possesses many of what was called by Chandrasekhar "miraculous" properties. Most of them can be related to the existence of a fundamental hidden symmetry called the principal conformal Killing-Yano (PCKY) tensor. In my talk I shall demonstrate that, in this context, four dimensions are not exceptional and that the (spherical horizon topology) higher-dimensional rotating black holes are very similar to their four-dimensional cousins. Namely, I shall present the most general spacetime admitting the PCKY tensor and show that is possesses the following properties: 1) it is of the algebraic type D, 2) it allows a separation of variables for the Hamilton-Jacobi, Klein-Gordon, Dirac, gravitational, and stationary string equations, 3) the geodesic motion in such a spacetime is completely integrable, 4) when the Einstein equations with the cosmological constant are imposed the metric becomes the Kerr-NUT-(A)dS spacetime. Some of these properties remain valid even when one includes the electromagnetic field.