@article{Walker_1973,
    doi = {10.1016/s0022-0000(73)80032-7},
    url = {https://doi.org/10.1016%2Fs0022-0000%2873%2980032-7},
    year = {1973},
    month = {aug},
    publisher = {Elsevier {BV}},
    volume = {7},
    number = {4},
    pages = {404--447},
    author = {S.A. Walker and H.R. Strong},
    title = {Characterizations of flowchartable recursions},
    journal = {Journal of Computer and System Sciences}
}

@article{Walker_1973, doi = {10.1016/s0022-0000(73)80032-7}, url = {https://doi.org/10.1016%2Fs0022-0000%2873%2980032-7}, year = {1973}, month = {aug}, publisher = {Elsevier {BV}}, volume = {7}, number = {4}, pages = {404--447}, author = {S.A. Walker and H.R. Strong}, title = {Characterizations of flowchartable recursions}, journal = {Journal of Computer and System Sciences} }