1. C. Berge, Graphs and Hypergraphs, 2nd ed., North-Holland, Amsterdam, 1976.
2. L. Boxer, A classical construction for the digital fundamental group, J. Math. Imaging Vis., 10 (1999), 51–62.
3. S.-E. Han, Non-product property of the digital fundamental group, Information Sciences 171(1-3) (2005) 73-91.
4. S.-E. Han, On the simplicial complex stemmed from a digital graph, Honam Mathematical Journal, 27 (2005), 115–129.
5. S.-E. Han, An equivalent property of a normal adjacency of a digital product, Honam Mathematical Journal, 36 (2014), 199–215.