1. A. H. Clifford and G. B. Preston, Algebraic Theory of Semigroups, Vol. 1, Am. Math. Soc. (1967).
2. E. S. Lyapin, Semigroups [in Russian], Fizmatgiz, Moscow (1960).
3. A. V. Rukolaine, ?Characters of finite inverse semigroups,? Zap. Nauchn. Sem. Leningr. Otd. Mat. Inst. Akad. Nauk SSSR,71, 207?215 (1977).
4. A. V. Rukolaine, ?Center of the semigroup algebra of a finite inverse semigroup over the field of complex numbers,? Zap. Nauchn. Sem. Leningr. Otd. Mat. Inst. Akad. Nauk SSSR,75, 154?158 (1978).
5. D. B. McAlister, ?Characters of finite semigroups,? J. Algebra,22, No. 1, 183?200(1972).