@article{RCM_1972__6_181590,
     author = {C. Ruiz Salguero and R. Ruiz and S. Frias},
     title = {On {Kann's} conditiion},
     journal = {Revista colombiana de matematicas},
     pages = {1--16},
     publisher = {mathdoc},
     volume = {6},
     year = {1972},
     zbl = {0257.55018},
     url = {http://geodesic.mathdoc.fr/item/RCM_1972__6_181590/}
}