@incollection{Koppenhagen_1997,
    doi = {10.1007/bfb0036189},
    url = {https://doi.org/10.1007%2Fbfb0036189},
    year = {1997},
    publisher = {Springer Berlin Heidelberg},
    pages = {257--268},
    author = {Ulla Koppenhagen and Ernst W. Mayr},
    title = {The complexity of the coverability, the containment, and the equivalence problems for commutative semigroups},
    booktitle = {Fundamentals of Computation Theory}
}

@incollection{Koppenhagen_1997, doi = {10.1007/bfb0036189}, url = {https://doi.org/10.1007%2Fbfb0036189}, year = {1997}, publisher = {Springer Berlin Heidelberg}, pages = {257--268}, author = {Ulla Koppenhagen and Ernst W. Mayr}, title = {The complexity of the coverability, the containment, and the equivalence problems for commutative semigroups}, booktitle = {Fundamentals of Computation Theory} }