@article{PismaZhETF.4.144,
    title = {Self-focusing and Focusing of Ultrasound and Hypersound},
    author = {Askar'yan, G. A.},
    journal = {Письма в ЖЭТФ},
    volume = {4},
    issue = {4},
    pages = {144},
    year = {1966},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_24855.shtml},
}