  In this paper, we study the problem of relaying a single bit over a tandem of
binary-input channels, with the goal of attaining the highest possible error
exponent in the exponentially decaying error probability. Our previous work
gave an exact characterization of the best possible error exponent in various
special cases, including when the two channels are identical, but the general
case was left as an open problem. We resolve this open problem by deriving a
new converse bound that matches our existing achievability bound.
