[2] E. Čech: 
Topological Spaces. Academia Praha, 1966. 
MR 0211373[3] F. Harary: 
Graph Theory. Addison-Wesley Publ. Соmр., Reading-Menlo Park-London- Don Mills, 1969. 
MR 0256911 | 
Zbl 0196.27202[4] Z. Hedrlín A. Pultr: 
On categorical embeddings of topological structures into algebraic. Comment. Math. Univ. Carolinae 7(1966), 377-400. 
MR 0202797[5] Z. Hedrlín A. Pultr V. Trnková: 
Concerning a categorical approach to topological and algebraic theories. Proc. 2nd Prague topo. Symposium, Academia Prague, 1966, 176-181. 
MR 0230784[6] S. Mac Lane: 
Categories for the Working Mathematician. Graduate Texts in Mathematics, Vol. 5, Springer-Verlag Berlin-Heidelberg-New York, 1971. 
MR 0354798 | 
Zbl 0232.18001[8] G. Sabidussi: Subdirect representations of graphs. Publ. of the Centre de Recherches Mathématiques, October 1973.
[9] J. Vinárek: 
Remarks on subdirect representations in categories. Comment. Math. Univ. Carolinae 19,1 (1978), 63-70. 
MR 0480670