@article{MZM_1984_36_4_a6,
     author = {V. G. Kurbatov and I. S. Frolov},
     title = {An operator variant of the maximum principle},
     journal = {Matemati\v{c}eskie zametki},
     pages = {531--535},
     year = {1984},
     volume = {36},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1984_36_4_a6/}
}