Harmonic maps and fundamental groups of nonpositively curved riemannian manifolds

Using the theory of harmonic maps the authors discuss the properties of the fundamental group of a complete nonpositively curved Riemannian manifold, and prove that the finitely generated virtual solvable subgroup of fundamental group of a complete nonpositively curved Riemannian manifold either is a peripheral subgroup of fundamental group or can be realized by an immersed totall geodesic closed flat manifold. It generalizes some results of Gromoll-Wolf, Lawson-Yan and Schoen-Yau.