@article{MZM_1970_7_4_a16,
     author = {V. A. Il'in},
     title = {Properties of spectra of nonnegative self-adjoint extensions of elliptic operators and exact conditions for the convergence and {Riesz} summability of {Fourier} series of various classes of functions},
     journal = {Matemati\v{c}eskie zametki},
     pages = {515--523},
     publisher = {mathdoc},
     volume = {7},
     number = {4},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1970_7_4_a16/}
}