A note on blowup limits in 3d Ricci flow (Q6042852)

The authors describe \(4\)-dimensional Ricci flows through singularities as metric flows, where a metric flow is given by a set \(X\), a time-function \(t:X\to \mathbb{R}\), complete separable metrics \(dt\) on the time-slices, and probability measures. Any metric flow \(X\) arising as a noncollapsed limit of smooth Ricci flows or arising from a \(3\)-dimensional singular Ricci flow is a weak solution of the Ricci flow. The authors prove that Perelman's ancient ovals occur as blowup limit in a \(3\)-dimensional Ricci flow through singularities if and only if there is an accumulation of spherical singularities.