@article{Rytter_1981,
    doi = {10.1016/0020-0190(81)90045-4},
    url = {https://doi.org/10.1016%2F0020-0190%2881%2990045-4},
    year = {1981},
    month = {jan},
    publisher = {Elsevier {BV}},
    volume = {13},
    number = {4-5},
    pages = {145--146},
    author = {Wojciech Rytter},
    title = {A hardest language recognized by two-way nondeterministic pushdown automata},
    journal = {Information Processing Letters}
}

@article{Rytter_1981, doi = {10.1016/0020-0190(81)90045-4}, url = {https://doi.org/10.1016%2F0020-0190%2881%2990045-4}, year = {1981}, month = {jan}, publisher = {Elsevier {BV}}, volume = {13}, number = {4-5}, pages = {145--146}, author = {Wojciech Rytter}, title = {A hardest language recognized by two-way nondeterministic pushdown automata}, journal = {Information Processing Letters} }