@article{Appel_1976,
    doi = {10.1090/s0002-9904-1976-14122-5},
    url = {https://doi.org/10.1090%2Fs0002-9904-1976-14122-5},
    year = {1976},
    publisher = {American Mathematical Society ({AMS})},
    volume = {82},
    number = {5},
    pages = {711--712},
    author = {K. Appel and W. Haken},
    title = {Every planar map is four colorable},
    journal = {Bulletin of the American Mathematical Society}
}

@article{Appel_1976, doi = {10.1090/s0002-9904-1976-14122-5}, url = {https://doi.org/10.1090%2Fs0002-9904-1976-14122-5}, year = {1976}, publisher = {American Mathematical Society ({AMS})}, volume = {82}, number = {5}, pages = {711--712}, author = {K. Appel and W. Haken}, title = {Every planar map is four colorable}, journal = {Bulletin of the American Mathematical Society} }