Channels with almost finite memory

A stationary channel can have two kinds of memory--input memory and output memory. Recently, Gray and Ornstein introduced a condition on the decay of input memory calledbar{d}-continuity and established coding theorems for d-continuous channels. In this paper a condition for the decay of output memory is introduced which is called conditional almost block independence (CABI). It is believed that these two properties describe the character of many channels of interest. The principal result states that thebar{d}-continuous CABI channels are precisely those which can be approximated in a very strong sense by a simple kind of finite memory channel, called a primitive channel, in which the source of randomness is memoryless and completely separated from the memory.