Complete totally geodesic subsets of the complex hyperbolic plane: an elementary classification

Abstract

The non-trivial complete totally geodesic submanifolds of the complex hyperbolic plane $\mathbb H_{\mathbb C}^2$ are the complex geodesics and the real planes. We present two new proofs for this fact. One is a short proof based on an algebraic formula for the Riemann curvature tensor due to S. Anan'in and C. Grossi and resembles the traditional proof using Lie theory. The other is purely elementary and geometric, relying on the structures in $\mathbb H_{\mathbb C}^2$ instead of general theories. In this second approach, we prove a slightly stronger result: the only non-trivial complete totally geodesic subsets of $\mathbb H_{\mathbb C}^2$ are the complex geodesics and the real planes without assuming that the subsets are submanifolds a priori. This second proof is also intriguing for only making use of elementary geometric constructions.