@article{DM_1989_1_2_a3,
     author = {L. P. Zhil'tsova},
     title = {Algorithmic complexity of problems of optimal alphabetical coding for context-free languages},
     journal = {Diskretnaya Matematika},
     pages = {38--51},
     publisher = {mathdoc},
     volume = {1},
     number = {2},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DM_1989_1_2_a3/}
}