@article{ZNSL_2009_372_a17,
     author = {S. S. Podkorytov},
     title = {On homotopy invariants of maps to the circle},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {187--202},
     publisher = {mathdoc},
     volume = {372},
     year = {2009},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2009_372_a17/}
}