[1]
Hildebrandt, S., Kaul, H. and Widman, K.- ove 1975. Harmonic mappings into Riemannian manifolds with non-positive sectional curvature. MATHEMATICA SCANDINAVICA. 37, (Dec. 1975), 257–263. DOI:https://doi.org/10.7146/math.scand.a-11604.