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    Approval compatible voting rules
    (Springer, 2026) Terzopoulou, Zoi; Lang, Jerome; Zwicker, William S.
    Suppose voters are asked to submit approval ballots for a certain set of alternatives, with approval voting applied to determine a winning alternative. The same voters are then asked to report rankings over these alternatives, and some voting rule intended for ranked ballots is applied. If voters are sincere, can an approval winner possibly win this second election? Can an approval loser lose that election, or all approval co-winners be co-winners of the election? These questions give rise to three notions of approval compatibility for voting rules: positive, negative, and uniform positive approval compatibility (PAC, NAC, and UPAC). We find that NAC is a very weak notion and UPAC is a very strong one. Moreover, PAC, a stronger variant of it called OPAC, and a weaker variant of UPAC called FUPAC divide usual voting rules into four families: Condorcet-consistent rules satisfy all of them; K-approval rules for K >= 2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$K\ge 2$$\end{document} satisfy none; plurality, plurality with runoff and STV satisfy OPAC but fail FUPAC; and Borda satisfies FUPAC and PAC but fails OPAC.
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    Pareto-efficiency of ordinal multiwinner voting rules
    (Springer Heidelberg, 2025) Laine, Jean; Lang, Jerome; Ozkal-Sanver, Ipek; Sanver, M. Remzi
    We investigate the Pareto-efficiency of ordinal multiwinner voting rules, that is, voting rules based on ordinal preference profiles over candidates. Defining Pareto-optimality of a committee requires relating the voters' rankings over individual candidates to their preferences over committees. We consider two well-known extension principles that extend rankings over candidates to preferences over committees: the responsive extension and the lexicographic extension. As the responsive extension outputs partial orders, we consider two Pareto-optimality notions: a committee is possibly (respectively, necessary) Pareto-optimal if it is Pareto-optimal for some (respectively, every) completion of these partial orders. As the lexicographic extension principle outputs a total order, it leads to only one Pareto-optimality notion. We then define several notions of Pareto-efficiency of multiwinner rules, depending on whether some (respectively, all) committees in the output are Pareto-optimal for one of the latter notions. We review what we believe to be a complete list of ordinal multiwinner rules that have been studied in the literature, and identify which Pareto-efficiency notions they satisfy. Our finding is that, somewhat surprisingly, these rules show a huge diversity: some satisfy the strongest notion, some do not even satisfy the weakest one, with many other rules at various intermediate levels.