Abstract. In this paper we prove that a constant mean curvature surface is proper-biharmonic in the unit Euclidean sphere S 4 if and only if it is minimal in a hypersphere S 3 (1/ √ 2).
IntroductionBiharmonic maps ϕ : (M, g) → (N, h) between Riemannian manifolds are critical points of the bienergy functionalwhere τ (ϕ) = trace ∇ dϕ is the tension field of ϕ that vanishes for harmonic maps (see [10]). The Euler-Lagrange equation corresponding to E 2 is given by the vanishing of the bitension fieldwhere J ϕ is formally the Jacobi operator of ϕ (see [14]). The operator J ϕ is linear, thus any harmonic map is biharmonic. We call proper-biharmonic the non-harmonic biharmonic maps. The study of proper-biharmonic submanifolds, i.e. submanifolds such that the inclusion map is non-harmonic (non-minimal) biharmonic, constitutes an important research direction in the theory of biharmonic maps. The first ambient spaces considered for their properbiharmonic submanifolds were the spaces of constant sectional curvature. Non-existence results were obtained for proper-biharmonic submanifolds in Euclidean and hyperbolic spaces (see [1, 4, 7, 9, 12]).The case of the Euclidean sphere is different. Indeed, the hypersphere S n−1 (1/ √ 2) and the generalized Clifford torus (1/ √ 2), n 1 + n 2 = n − 1, n 1 = n 2 .2000 Mathematics Subject Classification: Primary 58E20.