@article{SM_1993_75_2_a0,
     author = {I. Yu. Popov},
     title = {The {Helmholtz} resonator and the~theory of operator extensions in a~space with indefinite metric},
     journal = {Sbornik. Mathematics},
     pages = {285--315},
     publisher = {mathdoc},
     volume = {75},
     number = {2},
     year = {1993},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1993_75_2_a0/}
}