1. Clifford, A.H. and Preston, G.B., The algebraic theory of semigroups, I. Amer. Math. Soc., Providence, 1961;
2. Ellis, R., A note on the continuity of the inverse, Proc. Amer. Math. Soc. 9(1957), 372–373.
3. Ljapin, E.S., Semigroups, Firmatgiz, Moscow, 1960; English transl., Transl. Math. Mon., v.3, Amer. Math. Soc., Providence, 1963.
4. Math. Centre Tracts;A.B. Paalman-de Miranda,1964
5. Owen, W.S., The Rees theorem for locally compact semigroups, Semigroup Forum, 6(1973), 133–152.