@article{10_1017_S0017089500001026,
     author = {Hall, T. E.},
     title = {On the natural ordering of -classes and of idempotents in a regular semigroup},
     journal = {Glasgow mathematical journal},
     pages = {167--168},
     year = {1970},
     volume = {11},
     number = {2},
     doi = {10.1017/S0017089500001026},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500001026/}
}