Morse Index Stability for Sequences of Sacks-Uhlenbeck Maps into a Sphere

TL;DR

This paper proves the Morse index plus nullity are upper semicontinuous for sequences of p-harmonic maps into spheres, with improved gradient estimates and analysis of neck regions around blow-up points.

Contribution

It introduces improved gradient estimates in neck regions and demonstrates that these regions do not affect the Morse index stability for p-harmonic maps into spheres.

Findings

Neck regions do not contribute to the second variation negativity.

Morse index plus nullity are upper semicontinuous under energy bounds.

Gradient estimates are improved in blow-up analysis.

Abstract

In this paper we consider sequences of $p$-harmonic maps, $p>2$, from a closed Riemann surface $\Sigma$ into the $n$-dimensional sphere $\mathbb{S}^n$ with uniform bounded energy. These are critical points of the energy $E_p(u) :=\int_\Sigma \left( 1+|{\nabla u}|^2\right)^{p/2} \ dvol_\Sigma.$ Our two main results are an improved pointwise estimate of the gradient in the neck regions around blow up points and the proof that the necks are asymptotically not contributing to the negativity of the second variation of the energy $E_p.$ This allows us, in the spirit of the paper of the first and second authors in collaboration with M. Gianocca {\em Morse index stability for critical points to conformally invariant Lagrangians}, to show the upper semicontinuity of the Morse index plus nullity for sequences of $p$-harmonic maps into a sphere.