Optimal quantum adversary lower bounds for ordered search

The goal of the ordered search problem is to find a particular item in an
ordered list of n items. Using the adversary method, Hoyer, Neerbek, and Shi
proved a quantum lower bound for this problem of (1/pi) ln n + Theta(1). Here,
we find the exact value of the best possible quantum adversary lower bound for
a symmetrized version of ordered search (whose query complexity differs from
that of the original problem by at most 1). Thus we show that the best lower
bound for ordered search that can be proved by the adversary method is (1/pi)
ln n + O(1). Furthermore, we show that this remains true for the generalized
adversary method allowing negative weights.