Channels which transmit letters of unequal duration

In Section I an inequality of McMillan (1956) implied by unique decipherability is generalized to channels whose letters have unequal costs and used to prove that no uniquely decipherable encoding can increase the capacity of such a channel. The same methods yield a proof of the equivalence of several definitions of capacity. In Section II the matching problem is considered, a code construction of Shannon (1948) is generalized as a partial answer, and some unsolved problems are described. Finally, in Section III some generalizations are given for channels capable of transmitting a denumerable infinity of distinct letters