http://www.cs.ox.ac.uk/seminars/ en-gb 360 Mon, 05 Oct 2026 23:46:40 GMT Mon, 05 Oct 2026 23:46:40 GMT Computational Social Choice http://blogs.law.harvard.edu/tech/rss/ http://www.cs.ox.ac.uk/seminars/2851.html <p>Efficiency in multiwinner voting is most naturally captured by Pareto-optimality (PO), yet this notion is computationally and structurally difficult to handle. We therefore study fractional Pareto-optimality (fPO), under which a committee may not be dominated even by a fractional committee, i.e., any convex combination of committees. fPO turns out to be a natural refinement of PO as it retains exactly those Pareto-optimal committees whose efficiency is robust under uniform cloning of candidates. Furthermore, fPO committees are guaranteed to exist and have strong structural properties. We present a characterization of fPO in terms of weighted utilitarian welfare maximization, which yields a polynomial-time algorithm for verifying fPO and shows that the set of fPO committees satisfies committee monotonicity and is connected under single-candidate swaps. Analyzing welfarist rules through the lens of fPO, we further uncover an incompatibility between fPO and equality-oriented objectives. Most notably, we show that proportional approval voting (PAV) violates fPO in the approval setting. We close by pinpointing preference domains, including various one-dimensional ones, on which PO and fPO collapse into one notion.</p> <br/>RH-Strachey Room Computational Social Choice oucl-seminar-Computational Social Choice-2851 Mon, 05 Oct 2026 23:46:40 GMT