  Clustering a signed graph means partitioning the vertices into sets
("clusters") so that every positive edge, and no negative edge, is within a
cluster. Clustering is not always possible; the obstruction is circles with
exactly one negative edge ("weakly negative circles"). The correlation
clustering problem is to cluster with the minimum number of edges that violate
the clustering rule, called $Q$. A lower bound is $w$, the maximum number of
edge-disjoint weakly negative circles. If every two such circles are edge
disjoint, then $Q=w$. We characterize signed graphs of this kind.
