Complex Network Analysis Techniques Open access Peer reviewed

Is It Easier to Count Communities Than Find Them?

Cynthia Rush, Fiona Skerman, Alexander S. Wein, Dana Yang

Random Structures and Algorithms | Jun 9, 2026

Abstract

Abstract

ABSTRACT Random graph models with community structure have been studied extensively in the literature. For both the problems of detecting and recovering community structure, an interesting landscape of statistical and computational phase transitions has emerged. A natural unanswered question is: Might it be possible to infer properties of the community structure (for instance, the number and sizes of communities) even in situations where actually finding those communities is believed to be computationally hard? We show the answer is no. In particular, we consider certain hypothesis testing problems between models with different community structures, and we show (in the low‐degree polynomial framework) that testing between two options is as hard as finding the communities. Our methods give the first computational lower bounds for testing between two different “planted” distributions, whereas previous results have considered testing between a planted distribution and an i.i.d. “null” distribution. We also show a formal relationship between the low‐degree frameworks for recovery in a planted model and for testing two planted models.

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Authors

Researchers on this paper

Cynthia Rush

first | Columbia University | ORCID 0000-0001-6857-2855

Fiona Skerman

middle | Uppsala University | ORCID 0000-0003-4141-7059

Alexander S. Wein

middle | University of California, Davis | ORCID 0000-0002-3406-1747

Dana Yang

last | Cornell University | ORCID 0000-0001-9460-569X

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Citation

BibTeX

@article{Rush2026Easier,
  title = {Is It Easier to Count Communities Than Find Them?},
  author = {Cynthia Rush and Fiona Skerman and Alexander S. Wein and Dana Yang},
  journal = {Random Structures and Algorithms},
  year = {2026},
  doi = {10.1002/rsa.70071},
  url = {https://doi.org/10.1002/rsa.70071}
}

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