On the Morse index of branched Willmore spheres in 3-space

We develop a general method to compute the Morse index of branched Willmore spheres and show that the Morse index is equal to the index of certain matrix whose dimension is equal to the number of ends of the dual minimal surface (when the latter exists). As a corollary, we find that for all immersed Willmore spheres $$\vec {\Phi }:S^2\rightarrow \mathbb {R}^3$$ Φ → : S 2 → R 3 such that $$W(\vec {\Phi })=4\pi n$$ W ( Φ → ) = 4 π n , we have $$\mathrm {Ind}_{W}(\vec {\Phi })\le n-1$$ Ind W ( Φ → ) ≤ n - 1 .