On the Normal Stability of Triharmonic Hypersurfaces in Space Forms.

This article is concerned with the stability of triharmonic maps and in particular triharmonic hypersurfaces. After deriving a number of general statements on the stability of triharmonic maps we focus on the stability of triharmonic hypersurfaces in space forms, where we pay special attention to their normal stability. We show that triharmonic hypersurfaces of constant mean curvature in Euclidean space are weakly stable with respect to normal variations while triharmonic hypersurfaces of constant mean curvature in hyperbolic space are stable with respect to normal variations. For the case of a spherical target we show that the normal index of the small proper triharmonic hypersphere $\varphi :S^m(1/\sqrt{3})\hookrightarrow S^{m+1}$ is equal to one and make some comments on the normal stability of the proper triharmonic Clifford torus.