{"type":"ScholarlyArticle","title":"Ties in Multiwinner Approval Voting","authors":[{"name":"Łukasz Janeczko","slug":"ukasz-janeczko","given":"Łukasz","family":"Janeczko","affiliation":"AGH University , Poland"},{"name":"Piotr Faliszewski","slug":"piotr-faliszewski","given":"Piotr","family":"Faliszewski","affiliation":"AGH University , Poland"}],"abstract":"We study the complexity of deciding if there is a tie in a given approval-based multiwinner election, as well as the complexity of counting tied winning committees. We consider a family of Thiele rules, their greedy variants, Phragmen's sequential rule, and Method of Equal Shares. For most cases, our problems are computationally hard, but for sequential rules we find an FPT algorithm for discovering ties (parameterized by the committee size). We also show experimentally that in elections of moderate size ties are quite frequent.","doi":"10.24963/ijcai.2023/308","url":"https://ijcai.org/proceedings/2023/308","pdf":"https://ijcai.org/proceedings/2023/0308.pdf","markdown":"https://ijcai.org/proceedings/2023/308.md","bibtex":"https://ijcai.org/proceedings/2023/bibtex/308","volume":{"slug":"2023","title":"Proceedings of the Thirty-Second International Joint Conference on Artificial Intelligence","short_title":"IJCAI 2023","url":"https://ijcai.org/proceedings/2023/","year":2023,"location":"Macao, SAR","dates":"19-25 August 2023","editors":"Edith Elkind","isbn":"978-1-956792-03-4","publisher":"International Joint Conferences on Artificial Intelligence Organization","doi_prefix":"10.24963/ijcai.2023"},"section":"Game Theory and Economic Paradigms","track":"Main Track","number":308,"pages":{"first":2765,"last":2773},"keywords":["Game Theory and Economic Paradigms: GTEP: Computational social choice"],"videos":[],"references":[{"position":1,"raw":"[Aziz et al., 2015] H. 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