@article{PismaZhETF.5.238,
    title = {Collective Properties of Large-Radius Excitons},
    author = {Keldysh, L. V. and Kozlov, A. N.},
    journal = {πΙΣΨΝΑ Χ φότζ},
    volume = {5},
    issue = {7},
    pages = {238},
    year = {1967},
    doi = {},
    url = {http://jetpletters.ru/ps/0/article_25154.shtml},
}