  The work is devoted to study of quantum mutual information and coherent
information -- the two important characteristics of quantum communication
channel. Appropriate definitions of these quantities in the
infinite-dimensional case are given and their properties are studied in detail.
Basic identities relating quantum mutual information and coherent information
of a pair of complementary channels are proved. An unexpected continuity
property of quantum mutual information and coherent information following from
the above identities is observed. An upper bound for the coherent information
is obtained.
