## Tohoku Mathematical Journal 2017 March SECOND SERIES VOL. 69, NO. 1

 Tohoku Math. J. 69 (2017), 141-160

Title WILLMORE SURFACES IN SPHERES VIA LOOP GROUPS III: ON MINIMAL SURFACES IN SPACE FORMS

Author Peng Wang

(Received January 19, 2015, revised June 29, 2015)
Abstract. The family of Willmore immersions from a Riemann surface into $S^{n+2}$ can be divided naturally into the subfamily of Willmore surfaces conformally equivalent to a minimal surface in $\mathbb{R}^{n+2}$ and those which are not conformally equivalent to a minimal surface in $\mathbb{R}^{n+2}$. On the level of their conformal Gauss maps into $Gr_{1,3}(\mathbb{R}^{1,n+3})=SO^+(1,n+3)/SO^+(1,3)\times SO(n)$ these two classes of Willmore immersions into $S^{n+2}$ correspond to conformally harmonic maps for which every image point, considered as a 4-dimensional Lorentzian subspace of $\mathbb{R}^{1,n+3}$, contains a fixed lightlike vector or where it does not contain such a constant lightlike vector''. Using the loop group formalism for the construction of Willmore immersions we characterize in this paper precisely those normalized potentials which correspond to conformally harmonic maps containing a lightlike vector. Since the special form of these potentials can easily be avoided, we also precisely characterize those potentials which produce Willmore immersions into $S^{n+2}$ which are not conformal to a minimal surface in $\mathbb{R}^{n+2}$. It turns out that our proof also works analogously for minimal immersions into the other space forms.

Mathematics Subject Classification. Primary 53A30; Secondary 58E20; 53C43; 53C35.

Key words and phrases. Willmore surfaces, normalized potential, minimal surfaces, Iwasawa decompositions.