@article{10_4064_cm_31_2_223_229,
     author = {Udo Simon},
     title = {On differential operators of second order on {Riemannian} manifolds with nonpositive curvature},
     journal = {Colloquium Mathematicum},
     pages = {223--229},
     publisher = {mathdoc},
     volume = {31},
     number = {2},
     year = {1974},
     doi = {10.4064/cm-31-2-223-229},
     language = {en},
     url = {http://geodesic.mathdoc.fr/articles/10.4064/cm-31-2-223-229/}
}