@article{SIGMA_2021_17_a76,
     author = {Dmitri R. Yafaev},
     title = {Second-Order {Differential} {Operators} in the {Limit} {Circle} {Case}},
     journal = {Symmetry, integrability and geometry: methods and applications},
     year = {2021},
     volume = {17},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SIGMA_2021_17_a76/}
}