@article{10_1017_S0017089500007904,
     author = {Petrich, Mario and Rankin, Stuart A.},
     title = {Harmonic inverse semigroups},
     journal = {Glasgow mathematical journal},
     pages = {335--351},
     year = {1989},
     volume = {31},
     number = {3},
     doi = {10.1017/S0017089500007904},
     url = {http://geodesic.mathdoc.fr/articles/10.1017/S0017089500007904/}
}