Volume 2 (2006)
Article 13 pp. 249-266
Correlation Clustering with a Fixed Number of Clusters
Received: October 18, 2005
Published: October 22, 2006
Published: October 22, 2006
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Keywords: clustering, approximation algorithm, random sampling, polynomial time approximation scheme
Categories: algorithms, approximation algorithms, polynomial time approximation scheme, random sampling, clustering
ACM Classification: F.2.2, G.1.2, G.1.6
AMS Classification: 68W25, 05C85
Abstract: [Plain Text Version]
We continue the investigation of problems concerning
correlation clustering or clustering with qualitative
information, which is a clustering formulation that has been
studied recently (Bansal, Blum, Chawla (2004), Charikar, Guruswami,
Wirth (FOCS'03), Charikar, Wirth (FOCS'04), Alon et al. (STOC'05)).
In this problem, we are given a complete graph on n nodes (which
correspond to nodes to be clustered) whose edges are labeled +
(for similar pairs of items) and - (for dissimilar pairs of
items). Thus our input consists of only qualitative information on
similarity and no quantitative distance measure between items. The
quality of a clustering is measured in terms of its number of
agreements, which is simply the number of edges it correctly
classifies, that is the sum of number of - edges whose endpoints
it places in different clusters plus the number of + edges both of
whose endpoints it places within the same cluster.
In this paper, we study the problem of finding clusterings that maximize
the number of agreements, and the complementary minimization
version where we seek clusterings that minimize the number of
disagreements. We focus on the situation when the number of
clusters is stipulated to be a small constant k. Our main
result is that for every k, there is a polynomial time
approximation scheme for both maximizing agreements and minimizing
disagreements. (The problems are NP-hard for every k ≥ 2.)
The main technical work is for the minimization version, as the PTAS
for maximizing agreements follows along the lines of the property
tester for Max k-CUT by Goldreich, Goldwasser, Ron (1998).
In contrast, when the number of clusters is not specified, the problem
of minimizing disagreements was shown to be APX-hard (Chawla, Guruswami,
Wirth (FOCS'03)), even though the maximization version admits a PTAS.

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