  Given a sequence A of 2n real numbers, the Even-Rank-Sum problem asks for the
sum of the n values that are at the even positions in the sorted order of the
elements in A. We prove that, in the algebraic computation-tree model, this
problem has time complexity \Theta(n log n). This solves an open problem posed
by Michael Shamos at the Canadian Conference on Computational Geometry in 2008.
