@article{PismaZhETF.90.639,
    title = {Topological invariant  for superfluid  $^{\bf3}$He-B and quantum phase transitions},
    author = {Volovik, G. E.},
    journal = {πΙΣΨΝΑ Χ φότζ},
    volume = {90},
    issue = {8},
    pages = {639},
    year = {2009},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_28647.shtml},
}