@article{Valdes_1982,
    doi = {10.1137/0211023},
    url = {https://doi.org/10.1137%2F0211023},
    year = {1982},
    month = {may},
    publisher = {Society for Industrial {\&} Applied Mathematics ({SIAM})},
    volume = {11},
    number = {2},
    pages = {298--313},
    author = {Jacobo Valdes and Robert E. Tarjan and Eugene L. Lawler},
    title = {The Recognition of Series Parallel Digraphs},
    journal = {{SIAM} Journal on Computing}
}

@article{Valdes_1982, doi = {10.1137/0211023}, url = {https://doi.org/10.1137%2F0211023}, year = {1982}, month = {may}, publisher = {Society for Industrial {\&} Applied Mathematics ({SIAM})}, volume = {11}, number = {2}, pages = {298--313}, author = {Jacobo Valdes and Robert E. Tarjan and Eugene L. Lawler}, title = {The Recognition of Series Parallel Digraphs}, journal = {{SIAM} Journal on Computing} }