@incollection{Pol_k_2001,
    doi = {10.1007/3-540-44683-4_53},
    url = {https://doi.org/10.1007%2F3-540-44683-4_53},
    year = {2001},
    publisher = {Springer Berlin Heidelberg},
    pages = {611--620},
    author = {Libor Pol{\'{a}}k},
    title = {Syntactic Semiring of a Language},
    booktitle = {Mathematical Foundations of Computer Science 2001}
}

@incollection{Pol_k_2001, doi = {10.1007/3-540-44683-4_53}, url = {https://doi.org/10.1007%2F3-540-44683-4_53}, year = {2001}, publisher = {Springer Berlin Heidelberg}, pages = {611--620}, author = {Libor Pol{\'{a}}k}, title = {Syntactic Semiring of a Language}, booktitle = {Mathematical Foundations of Computer Science 2001} }