@article{Atserias_2006,
    doi = {10.1145/1131342.1131344},
    url = {https://doi.org/10.1145%2F1131342.1131344},
    year = {2006},
    month = {mar},
    publisher = {Association for Computing Machinery ({ACM})},
    volume = {53},
    number = {2},
    pages = {208--237},
    author = {Albert Atserias and Anuj Dawar and Phokion G. Kolaitis},
    title = {On preservation under homomorphisms and unions of conjunctive queries},
    journal = {Journal of the {ACM}}
}

@article{Atserias_2006, doi = {10.1145/1131342.1131344}, url = {https://doi.org/10.1145%2F1131342.1131344}, year = {2006}, month = {mar}, publisher = {Association for Computing Machinery ({ACM})}, volume = {53}, number = {2}, pages = {208--237}, author = {Albert Atserias and Anuj Dawar and Phokion G. Kolaitis}, title = {On preservation under homomorphisms and unions of conjunctive queries}, journal = {Journal of the {ACM}} }