LAUSR

In this paper, we prove that the class of bi-f-harmonic maps and that of f-biharmonic maps from a conformal manifold of dimension $$\ge 3$$≥3 are the same (Theorem 1.1). We also give several results on nonexistence of proper bi-f-harmonic maps and f-biharmonic maps from complete Riemannian manifolds into nonpositively curved Riemannian manifolds. These include: any bi-f-harmonic map from a compact manifold into a non-positively curved manifold is f-harmonic, and any f-biharmonic (respectively, bi-f-harmonic) map with bounded f and bounded f-bienergy (respectively, bi-f-energy) from a complete Riemannian manifold into a negatively curved manifold has rank $$\le 1$$≤1 everywhere.