@article{701578,
     title = {A high-order helicity invariant and the Rokhlin theorem},
     booktitle = {Proceedings of the 15th Winter School "Geometry and Physics"},
     series = {GDML\_Books},
     publisher = {Circolo Matematico di Palermo},
     address = {Palermo},
     year = {1996},
     pages = {[85]-94},
     mrnumber = {MR1463511},
     zbl = {0897.57008},
     url = {http://dml.mathdoc.fr/item/701578}
}

Akhmetiev, Peter M. A high-order helicity invariant and the Rokhlin theorem, dans Proceedings of the 15th Winter School "Geometry and Physics", GDML_Books,  (1996), pp. [85]-94. http://gdmltest.u-ga.fr/item/701578/