Stability of the (two-loop) Renormalization Group Flow for Nonlinear   Sigma Models
  We prove the stability of the torus, and with suitable rescaling, hyperbolic space under the (two-loop) renormalization group flow for the nonlinear sigma model. To prove stability we use similar techniques to \cite{GIK02}, where the stability of the torus under Ricci flow was first established. The main technical tool is maximal regularity theory. 
Christine Guenther and Todd A. Oliynyk