@article{SF_1979__18_134357,
     author = {K. Todorov},
     title = {Inverse elements and their average number in a finite symmetric semigroup.},
     journal = {Semigroup forum},
     pages = {279--281},
     publisher = {mathdoc},
     volume = {18},
     year = {1979},
     zbl = {0419.20052},
     url = {http://geodesic.mathdoc.fr/item/SF_1979__18_134357/}
}