  Here we write in a unified fashion (using "R(P, Q, D)") the random coding
exponents in channel coding and lossy source coding. We derive their explicit
forms and show, that, for a given random codebook distribution Q, the channel
decoding error exponent can be viewed as an encoding success exponent in lossy
source coding, and the channel correct-decoding exponent can be viewed as an
encoding failure exponent in lossy source coding. We then extend the channel
exponents to arbitrary D, which corresponds for D > 0 to erasure decoding and
for D < 0 to list decoding. For comparison, we also derive the exact random
coding exponent for Forney's optimum tradeoff decoder.
