Sequential Apportionment from Stationary Divisor Methods

TL;DR

This paper analyzes stationary divisor apportionment methods, characterizing their sequences, extending results to multiple parties, and revealing new insights into size bias and relationships between classical methods.

Contribution

It provides a comprehensive characterization of apportionment sequences for stationary divisor methods and connects classical divisor methods through sequence analysis.

Findings

Sequences are periodic for integer votes.

A systematic extension to n-party settings is developed.

New relationships between Adams and D'Hondt methods are identified.

Abstract

Divisor methods are well known to satisfy house monotonicity, which allows representative seats to be allocated sequentially. We focus on stationary divisor methods defined by a rounding cutpoint $c \in [0,1]$. For such methods with integer-valued votes, the resulting apportionment sequences are periodic. Restricting attention to two-party allocations, we characterize the set of possible sequences and establish a connection between the lexicographical ordering of these sequences and the parameter $c$. We then show how sequences for all pairs of parties can be systematically extended to the $n$-party setting. Further, we determine the number of distinct sequences in the $n$-party problem for all $c$. Our approach offers a refined perspective on size bias: rather than viewing large parties as simply receiving more seats, we show that they instead obtain their seats earlier in the apportionment sequence. Of particular interest is a new relationship we uncover between the sequences generated by the smallest divisor (Adams) and greatest divisor (D'Hondt or Jefferson) methods.