# An Ω lower bound for computing the sum of even-ranked elements

Given a sequence A of 2n real numbers, the \ers\ 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.

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Mörig, Marc / Rautenbach, Dieter / Smid, Michiel / et al: An Ω lower bound for computing the sum of even-ranked elements. 2009.

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