On the normal stability of the 4-harmonic and the ES-4-harmonic hypersphere

Abstract: Both 4-harmonic and ES-4-harmonic maps are two higher order generalizations of the well-studied harmonic map equation given by a nonlinear elliptic partial differential equation of order eight. Due to the large number of derivatives it is very difficult to find any difference in the qualitative behavior of these two variational problems. It is well known that the small hypersphere (\iota\colon\s^m(\frac{1}{2})\to\s^{m+1}) is a critical point of both the 4-energy as well as the ES-4-energy but up to now it has not been investigated if there is a difference concerning its stability. The main contribution of this article is to show that the small hypersphere is unstable with respect to normal variations both as 4-harmonic hypersphere as well as ES-4-harmonic hypersphere and that its normal index equals one in both cases.