@article{DE_1991_27_12_a26,
     author = {M. I. Sumin},
     title = {On the first variation in the theory of optimal control of systems with distributed parameters},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2179--2181},
     publisher = {mathdoc},
     volume = {27},
     number = {12},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_12_a26/}
}