@article{DE_1991_27_7_a16,
     author = {S. I. Piskarev},
     title = {Approximation of positive $C_0$-semigroups of operators},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1245--1250},
     publisher = {mathdoc},
     volume = {27},
     number = {7},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_7_a16/}
}