@article{PismaZhETF.28.559,
    title = {Applicability of the Fermat principle to the optics of ultracold neutrons},
    author = {Frank, I. M. and Frank, A. I.},
    journal = {πΙΣΨΝΑ Χ φότζ},
    volume = {28},
    issue = {8},
    pages = {559},
    year = {1978},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_24217.shtml},
}