Harmonic maps from $S^3$ into $S^2$ with low Morse index

Abstract

We prove that any smooth harmonic map from $S^3$ into $S^2$ of Morse index less or equal than $4$ has to be an harmonic morphism, that is the successive composition of an isometry of $S^3$, the Hopf fibration and an holomorphic map from $\mathbb{C}P^1$ into itself.