@article{IM2_1985_25_2_a6,
     author = {M. A. Chahkiev},
     title = {Linear differential operators with real spectrum, and optimal quadrature formulas},
     journal = {Izvestiya. Mathematics },
     pages = {391--417},
     publisher = {mathdoc},
     volume = {25},
     number = {2},
     year = {1985},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/IM2_1985_25_2_a6/}
}