To begin a small interest in Ricci Flow and similar tools, I am starting with Hamilton's expository paper *The Formation of Singularities in the Ricci Flow*. This was posted in 1995, so I am wondering if some of his "intuitive pictures" can now be made rigorous:

Consider a dumbbell metric on $S^3$, where the neck looks like $S^2\times B^1$ (something like this, except that picture is for $S^2$). The positive curvature at the ends of the dumbbell will cause the metric to contract (rounding out those parts), whereas the neck area should shrink. In particular, the neck should pinch off at some time $t_{pinch}>0$ (topologically a wedge-sum $S^3\vee S^3$). **Has this been put into experimental practice?** More to the point, Hamilton remarks that *there may be a weak solution extending past the pinching moment when the sphere splits into two spheres, although weak solutions haven't been defined for the Ricci flow*. **To what extent can we currently rigorous this in practice (if at all)?**

weaksolutions, this pinching problem is precisely what Perelman's Ricci flow with surgery is designed to dodge. – Benoît Kloeckner Dec 23 '12 at 11:02