Published online by Cambridge University Press: 27 April 2020
A Willmore surface   $y:M\rightarrow S^{n+2}$  has a natural harmonic oriented conformal Gauss map
 $y:M\rightarrow S^{n+2}$  has a natural harmonic oriented conformal Gauss map   $Gr_{y}:M\rightarrow SO^{+}(1,n+3)/SO(1,3)\times SO(n)$ , which maps each point
 $Gr_{y}:M\rightarrow SO^{+}(1,n+3)/SO(1,3)\times SO(n)$ , which maps each point   $p\in M$  to its oriented mean curvature 2-sphere at
 $p\in M$  to its oriented mean curvature 2-sphere at   $p$ . An easy observation shows that all conformal Gauss maps of Willmore surfaces satisfy a restricted nilpotency condition, which will be called “strongly conformally harmonic.” The goal of this paper is to characterize those strongly conformally harmonic maps from a Riemann surface
 $p$ . An easy observation shows that all conformal Gauss maps of Willmore surfaces satisfy a restricted nilpotency condition, which will be called “strongly conformally harmonic.” The goal of this paper is to characterize those strongly conformally harmonic maps from a Riemann surface   $M$  to
 $M$  to   $SO^{+}(1,n+3)/SO^{+}(1,3)\times SO(n)$ , which are the conformal Gauss maps of some Willmore surface in
 $SO^{+}(1,n+3)/SO^{+}(1,3)\times SO(n)$ , which are the conformal Gauss maps of some Willmore surface in   $S^{n+2}.$  It turns out that generically, the condition of being strongly conformally harmonic suffices to be associated with a Willmore surface. The exceptional case will also be discussed.
 $S^{n+2}.$  It turns out that generically, the condition of being strongly conformally harmonic suffices to be associated with a Willmore surface. The exceptional case will also be discussed.
The second author is thankful to the ERASMUS MUNDUS TANDEM Project for the financial supports to visit the TU München.
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               : transforms and vanishing theorems, dissertation, Technische Universität Berlin, 2005.Google Scholar
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               : transforms and vanishing theorems, dissertation, Technische Universität Berlin, 2005.Google Scholar