On the Succinct Representation of Equivalence Classes

Abstract

Given a set of n elements that are partitioned into equivalence classes, we study the problem of assigning unique labels to these elements in order to support the query that asks whether the elements corresponding to two given labels belong to the same equivalence class. This problem has been studied by Katz et al., Alstrup et al., and Lewenstein et al.. Lewenstein et al. showed that with no auxiliary data structure, a label space of size nlg(n) is necessary and sufficient to represent the equivalence relation. They also showed that if the labels were to be assigned from the set [n], a data structure of square root of n bits is necessary and sufficient to represent the equivalence relation and to answer the equivalence query in O(lg(n)) time. In this thesis, we give an improved data structure that uses O(square root of n) bits and can answer queries in constant time, when the label space is of size n. Moreover, we study the case where we allow the label space to be of size cn for any constant c > 1. We show that with such a label space, a data structure of O(lg(n)) bits is necessary and sufficient to represent the equivalence relation and to answer the equivalence query in constant time. We believe that our work can trigger further work on tradeoffs between label space and auxiliary data structure space for other labeling problems.