@article{ZNSL_2001_279_a1,
     author = {P. M. Akhmet'ev},
     title = {A higher-order analog of the helicity number for a~pair of divergent-free vector fields},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {15--23},
     publisher = {mathdoc},
     volume = {279},
     year = {2001},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_2001_279_a1/}
}