Stability of the Morse Index for the $p$-harmonic Approximation of Harmonic Maps into Homogeneous Spaces

Abstract

In the joint work of the author with Da Lio and Rivi\`ere (Morse Index Stability for Sequences of Sacks-Uhlenbeck Maps into a Sphere) we studied the stability of the Morse index for Sacks-Uhlenbeck sequences into spheres as $p\searrow2$. These are critical points of the energy $E_p(u) := \int_\Sigma \left( 1+|\nabla u|^2\right)^{p/2} \ dvol_\Sigma,$ where $u:\Sigma \rightarrow S^n$ is a map from a closed Riemannian surface $\Sigma$ into a sphere $ S^n$. In this paper we extend the results found in our previous work to the case of Sacks-Uhlenbeck sequences into homogeneous spaces, by incorporating the strategy introduced by Bayer and Roberts (Energy identity and no neck property for $\epsilon$-harmonic and $\alpha$-harmonic maps into homogeneous target manifolds). In the spirit of the work of Da Lio, Gianocca and Rivi\`ere (Morse Index Stability for Critical Points to Conformally invariant Lagrangians), we show in this setting the upper semicontinuity of the Morse index plus nullity and an improved pointwise estimate of the gradient in the neck regions around blow up points.