[1] A. H. Clifford, D. D. Miller: 
Union and symmetry preserving endomorphisms of the semigroup of all binary relations on a set. Czechoslovak Math. Journ. 20 (1970), 303 - 314. 
MR 0262395[2] A. H. Clifford, G. B. Preston: 
The algebraic theory of semigroups, Vol. 1. Amer. Math. Soc., Math. Surveys No. 7, 1961. 
MR 0132791[3] R. Engelking: 
Outline of general topology. North-Holland Publishing Co., Amsterdam, Polish Scientific Publishers, 1968. 
MR 0230273 | 
Zbl 0157.53001[4] J. de Groot: 
Topological characterization of all subsets of the real number system. Nederl. Akad. Wetensch. Proc. 50 (1947), 876-884. 
MR 0022665 | 
Zbl 0029.23203[8] K. D. Magill, Jr., S. Yamamura: Union and symmetry preserving homomorphisms of semigroups of closed relations. submitted to the Proc. of the London Math. Soc.
[9] G. T. Whyburn: 
Analytic Topology. Colloquium Publications, Vol. XXVIII, Amer. Math. Soc., New York, 1942. 
MR 0007095 | 
Zbl 0061.39301