@article{SM_2013_204_4_a4,
     author = {A. A. Ilyin},
     title = {Lower bounds for sums of eigenvalues of elliptic operators and systems},
     journal = {Sbornik. Mathematics},
     pages = {563--587},
     year = {2013},
     volume = {204},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2013_204_4_a4/}
}