Perpetually Fair Assignments Via Balanced Sequences of Permutations

Abstract

There is a set of $n$ indivisible items (goods or chores), and a set of $n$ players. Each day, a single item should be assigned to each player. Assignments based on latin squares guarantee fairness after every $n$ days; our goal is to ensure fairness after every single day. We present two 'balance' conditions on latin squares. Informally, a latin square is balanced if its top rows and leftmost columns contain all $n$ labels; this ensures that all $n$ players receive one of the top items in one of the early days. One such condition can always be satisfied, but is arguably too weak; a second condition is strong, and can be satisfied for all $n\le 12$, but cannot be satisfied for some larger values of $n$, including all $n>108$. We show that the second balance condition guarantees that the cumulative assignment is always \emph{proportional up to one item (PROP1)}, where proportionality holds in a strong ordinal sense -- for every valuations that are consistent with the item ranking. Finally, we present a weaker balance condition on a sequence, that guarantees ordinal proportionality up to two items (PROP2). Whether or not this condition can be satisfied for all $n$ remains an open question.

Full version of a paper accepted to SAGT 2026 conference