1. C. Berge, Graphs and hypergraphs, 2nd edition, North-Holland, Amsterdam, 1976.
2. K. Borsuk, Theory of retracts, Polish Scientific Publishers, Warsaw, 1967.
3. Digitally continuous functions
4. A classical construction for the digital fundamental group;Boxer;Pattern Recognit Lett,1999
5. Properties of Digital Homotopy