Strengthening Proportionality in Temporal Voting

We study proportional representation in the framework of temporal voting with approval ballots. Prior work adapted basic proportional representation concepts---justified representation (JR), proportional JR (PJR), and extended JR (EJR)---from the multiwinner setting to the temporal setting. Our work introduces and examines ways of going beyond EJR. Specifically, we consider stronger variants of JR, PJR, and EJR, and introduce temporal adaptations of more demanding multiwinner axioms, such as EJR+, full JR (FJR), full proportional JR (FPJR), and core stability. For each of these concepts, we investigate its existence and study its relationship to existing notions, thereby establishing a rich hierarchy of proportionality concepts. Notably, we show that two of our proposed axioms---EJR+ and FJR---strengthen EJR while remaining satisfiable in every temporal election.