@article{DAN_1963_151_6_a8,
     author = {F. I. Karpelevich},
     title = {Non-negative eigenfunctions of the {Beltrami{\textendash}Laplace} operator on symmetric spaces of non-positive curvature},
     journal = {Doklady Akademii Nauk},
     pages = {1274--1276},
     year = {1963},
     volume = {151},
     number = {6},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1963_151_6_a8/}
}