@article{PismaZhETF.9.683,
    title = {Region of Positive Existence of 2S Positronium in a Medium},
    author = {Urbanovich, S. I.},
    journal = {πΙΣΨΝΑ Χ φότζ},
    volume = {9},
    issue = {12},
    pages = {683},
    year = {1969},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_25428.shtml},
}