On polar actions invariant solutions of semi-linear equations on manifolds

In this paper we put together some tools from differential geometry and analysis to study second order semi-linear partial differential equations on a Riemannian manifold M. We look for solutions that are constants along orbits of a given group action. Using some results obtained by Helgason in [10] we are able to write a (reduced) second order semi-linear problem on a submanifold Sigma. This submanifold is, in a sense, transversal to the orbits of the group actions and its existence is assumed. We describe precise conditions on the Riemannian Manifold M and the submanifold Sigma in order to be able to write the reduced equation on Sigma. These conditions are satisfied by several particular cases including some examples treated separately in the literature such as the sphere, surfaces of revolution and others. Our framework also includes the setup of polar actions or exponential coordinates. Using this procedure, we are left with a second order semi-linear equation posed on a submanifold. In particular, if the submanifold Sigma is one-dimensional, we can use suitable tools from analysis to derive existence and properties of solutions. (C) 2019 Elsevier Inc. All rights reserved.