Classification of singular radial solutions to the σ_k Yamabe equation on annular domains

The study of the kth elementary symmetric function of the Weyl-Schouten curvature tensor of a Riemannian metric, the so-called σ_k curvature, has produced many fruitful results in conformal geometry in recent years. In these studies in conformal geometry, the deforming conformal factor is considered to be a solution of a fully nonlinear elliptic PDE. Important advances have been made in recent years in the understanding of the analytic behavior of solutions of the PDE. However, the singular behavior of these solutions, which is important in describing many important questions in conformal geometry, is little understood. This note classifies all possible radial solutions, in particular, the singular solutions of the σ_k Yamabe equation, which describes conformal metrics whose σ_k curvature equals a constant. Although the analysis involved is of elementary nature, these results should provide useful guidance in studying the behavior of singular solutions in the general situation.