We consider topological
Markov chains (also called Markov shifts) on countable graphs. We show that a
transient graph can be extended to a recurrent graph of equal entropy which is either
positive recurrent of null recurrent, and we give an example of each type.
We extend the notion of local entropy to topological Markov chains and
prove that a transitive Markov chain admits a measure of maximal entropy
(or maximal measure) whenever its local entropy is less than its (global)
entropy.