  We derive a single-letter upper bound to the mismatched-decoding capacity for
discrete memoryless channels. The bound is expressed as the mutual information
of a transformation of the channel, such that a maximum-likelihood decoding
error on the translated channel implies a mismatched-decoding error in the
original channel. In particular, a strong converse is shown to hold for this
upper-bound: if the rate exceeds the upper-bound, the probability of error
tends to 1 exponentially when the block-length tends to infinity. We also show
that the underlying optimization problem is a convex-concave problem and that
an efficient iterative algorithm converges to the optimal solution. In
addition, we show that, unlike achievable rates in the literature, the
multiletter version of the bound does not improve. A number of examples are
discussed throughout the paper.
