Dutch Social Choice Colloquium

The next session of the Dutch Social Choice Colloquium is planned for Friday, 25 October 2019 in Amsterdam. We hope to see you there!

You may also be interested in the Conference in Honour of Hans Peters on Social Choice, Game Theory, and Applications to be held in Maastricht on 30-31 January 2020.

**Speakers:**
Piotr Faliszewski,
Boas Kluiving,
Hans Peters

**Location:** Belle van Zuylenzaal (C1.13), University Library, Singel 425, Amsterdam (directions)

**Host:** Ulle Endriss (University of Amsterdam)

**Programme:**

14:00-14:45 | Piotr Faliszewski (AGH University of Science and Technology, Krakow) How to Mix Multiwinner Voting Rules and Why? Abstract:
There are three idealized types of outcomes of multiwinner voting
rules. The committees may either consist of individually
excellent candidates (as needed, e.g., when choosing finalists of
competitions), may be diverse (e.g., when we look for products to
offer on a homepage of an Internet store), or may be
proportional (e.g., when we want to form a parliament). However,
in reality we may often be interested in committees that achieve
various levels of compromise between these notions. For example,
in degressively proportional parliamentary elections we want to
increase the representation of parties with smaller support, at
the expense of those with larger support. As a consequence,
degressive proportionality can be seen as mixing the ideals of
proportionality and diversity. In this talk I will discuss
various ways of forming multiwinner voting rules that achieve
such compromises and discuss their computational complexity. I
will also show that the notion of proportionality is much more
tricky than one might suspect. |

14:45-15:15 | Coffee Break |

15:15-16:00 | Boas Kluiving (University of Amsterdam) Analysing Irresolute Multiwinner Voting Rules via SAT Solving Abstract: Suppose we want to elect a committee or parliament by using a voting rule under which each voter is asked to approve of a subset of the candidates. There are several properties we may want such a voting rule to satisfy: it should ensure that voters do not have an incentive to misrepresent their preferences, that outcomes respect some form of proportional representation of the voters, and that outcomes are Pareto efficient. We show that it is impossible to design a voting rule that satisfies all three properties and explore what possibilities there are when we weaken our requirements. Of special interest is the methodology we use: part of the proof can be outsourced to a SAT (satisfiability) solver by translating an instance of the statement of our main theorem into a set of formulas in propositional logic. While prior work has considered similar questions for the special case of resolute voting rules, which do not allow for ties between outcomes, we focus on the fact that, in practice, most voting rules do allow for the possibility of such ties. This is joint work with Adriaan de Vries, Pepijn Vrijbergen, Arthur Boixel, and Ulle Endriss. |

16:00-16:30 | Coffee Break |

16:30-17:15 | Hans Peters (Maastricht University) Choosing k from mAbstract: We show that feasible elimination procedures (Peleg, 1978) can be used to select k from m alternatives. An application is the choice of a committee of size k from a set of m of available candidates. An important advantage of this method is the core property: no coalition of voters can guarantee an outcome that is preferred by all its members. We also show that the problem of determining whether a specific k-tuple can result from a feasible elimination procedure is computationally equivalent to the problem of finding a maximal matching in a bipartite graph. Additionally, we provide an axiomatic characterization of the method of feasible elimination. This presentation is based on joint work with Bezalel Peleg. |

17:30 | Drinks in a nearby café |