@article{SF_1983__26_134602,
     author = {T. Imaoka and Yamada},
     title = {Some remarks on the free regular *-semigroup generated by a single element.},
     journal = {Semigroup forum},
     pages = {191--204},
     year = {1983},
     volume = {26},
     zbl = {0513.20045},
     url = {http://geodesic.mathdoc.fr/item/SF_1983__26_134602/}
}