An Ω (n log n) lower bound for computing the sum of even-ranked elements

Given a sequence A of 2n real numbers, the EvenRankSum 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 Θ (n log n). This solves an open problem posed by Michael Shamos at the Canadian Conference on Computational Geometry in 2008