@inproceedings{Hopcroft_1967,
    doi = {10.1109/focs.1967.4},
    url = {https://doi.org/10.1109%2Ffocs.1967.4},
    year = {1967},
    publisher = {{IEEE}},
    author = {J. E. Hopcroft and J. D. Ullman},
    title = {An approach to a unified theory of automata},
    booktitle = {8th Annual Symposium on Switching and Automata Theory ({SWAT} 1967)}
}

@inproceedings{Hopcroft_1967, doi = {10.1109/focs.1967.4}, url = {https://doi.org/10.1109%2Ffocs.1967.4}, year = {1967}, publisher = {{IEEE}}, author = {J. E. Hopcroft and J. D. Ullman}, title = {An approach to a unified theory of automata}, booktitle = {8th Annual Symposium on Switching and Automata Theory ({SWAT} 1967)} }