[1] Clifford A. H., G. B. Preston: 
Algebraic theory of semigroups. Math. Survey vol. I. No. 7, Amer. Math. Soc. Providence RI(1961). 
MR 0132791 | 
Zbl 0111.03403[2] Clifford A. H.: 
The fundamental representation of a regular semigroup. Department of Maths, Tulane University (1974). 
MR 0393312[4] Meakin J.: 
The maximum idemptotent separating congruence on a regular semigroup. Proceedings of Edinburgh Math. Soc. Vol. 18, series II (1972-73) 159-162. 
DOI 10.1017/S0013091500009846 | 
MR 0318362[5] Nambooripad K. S. S.: 
Structure of regular semigroups I. Semigroup forum 9 (1974) 354- 363. 
MR 0376920 | 
Zbl 0298.20059[6] Zelinka В.: 
Tolerances in algebraic structures. Czech. Math. Journal Vol. 25 (1975) 175- 178. 
MR 0457323