Sparsifying Correlation Clustering: Edge Coresets, Triangle Witnesses, and Observation Lower Bounds

The standard metric LP for correlation clustering has $\Theta(n^2)$ pairwise marginals and $\Theta(n^3)$ triangle inequalities, while dense pairwise supervision is often the primary bottleneck in large relational pipelines. We separate three sparsification questions that are often treated together: preserving the objective of every integral clustering, rounding from a sparse set of LP marginals, and clustering when the signed weighted input itself is only sparsely observed. First, we prove that the VC dimension of the signed-edge disagreement class induced by all clusterings of $n$ vertices is exactly $n-1$, so weighted edge sampling yields additive $\varepsilon$-coresets of size $\tilde O(n/\varepsilon^2)$ with optimal $n$-dependence. Second, we introduce *Sparse-LP-Pivot*, which imputes missing LP marginals from triangle witnesses, and analyze it at two levels: a universal but coarse perturbation bound for every Lipschitz LP-PIVOT rule, and a sharper sparse-rounding theorem under an explicit weighted influence-stability certificate. For pseudometric-weighted CC, this gives a $10/3$ exact-marginal baseline and an unconditional coarse sparse-marginal bound; a sharp same-edge bound governed by $\overline{\Gamma}_w$ follows conditionally if the rounding rule is additionally proved to satisfy edge-weight-dominated influence stability. Third, we show that uniform marginal sampling has a triangle-witness threshold at $m=\Theta(n^{3/2})$ for typical pairs, while $m=O(n^{3/2}\sqrt{\log n})$ is sufficient for all pairs with high probability. Finally, in the stricter sparse edge-observation model, we prove an $\Omega(n^{3/2})$ lower bound on the expected approximation ratio from $o(n)$ uniformly sampled edges for general weighted instances. Experiments illustrate the witness transition and the proposed diagnostics; the formal guarantees are stated independently of those empirical observations.