@article{10_1017_S0017089500002081,
     author = {Cordes, Craig M.},
     title = {Semigroups of non-negative integral matrices},
     journal = {Glasgow mathematical journal},
     pages = {39--42},
     year = {1974},
     volume = {15},
     number = {1},
     doi = {10.1017/S0017089500002081},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500002081/}
}