[2] M. EL-ZAHAR N. SAUER:
The chromatic number of product of two 4-chromatic graphs is 4. Combinatorica 5 (1985), 121-126.
MR 0815577
[3] A. HAJNAL:
The chromatic number of the product of two, -chromatic graphs can be countable. Combinatorica 5 (1985), 137-139.
MR 0815579
[4] S. T. HEDETNIEMI: Homomorphisms of graphs and automata. Univ. of Michigan Technical Report 03 105-44-T, 1966