In this work, we study k-min-sum-of-radii (k-MSR) clustering under mergeable constraints. k-MSR seeks to group data points using a set of up to k balls, such that the sum of the radii of the balls is minimized. A clustering constraint is called mergeable if merging two clusters satisfying the constraint, results in a cluster that also satisfies the constraint. Many popularly studied constraints are mergeable, including fairness constraints and lower bound constraints. In our work, we design a (4+ε)-approximation for k-MSR under any given mergeable constraint with runtime 2^{O(k/(ε)⋅log²k/ε)} n⁴, i.e., fixed-parameter tractable in k for constant ε. Our result directly improves upon the FPT (6+ε)-approximation by Carta et al. [Carta et al., 2024]. We also provide a hardness result that excludes the exact solvability of k-MSR under any given mergeable constraint in time f(k)n^o(k), assuming ETH is true.
@InProceedings{bandyapadhyay_et_al:LIPIcs.APPROX/RANDOM.2025.23, author = {Bandyapadhyay, Sayan and Chen, Tianzhi}, title = {{Improved FPT Approximation for Sum of Radii Clustering with Mergeable Constraints}}, booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2025)}, pages = {23:1--23:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-397-3}, ISSN = {1868-8969}, year = {2025}, volume = {353}, editor = {Ene, Alina and Chattopadhyay, Eshan}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2025.23}, URN = {urn:nbn:de:0030-drops-243894}, doi = {10.4230/LIPIcs.APPROX/RANDOM.2025.23}, annote = {Keywords: sum-of-radii clustering, mergeable constraints, approximation algorithm} }
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