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Containing input symbols and G = g1 , g2 , . . . , gm , containing output symbols, where can also be m = n. Since we don't know which symbol fi generated the symbol gj , the characterization of the channel is given by the set P fi gj . Now we can define the mutual contagion I fi ; gj = P(fi |gj ) 1 1 log P f - log P f |g = log P(fi ) , where P(f i ) is an "a ( i) ( i j) priori" estimate. Thus: I f