# Manual Hamilton’s Ricci Flow

In particular, the result of geometrization may be a geometry that is not isotropic.

## Ricci flow - Wikipedia

In most cases including the cases of constant curvature, the geometry is unique. An important theme in this area is the interplay between real and complex formulations. In particular, many discussions of uniformization speak of complex curves rather than real two-manifolds. The Ricci flow does not preserve volume, so to be more careful, in applying the Ricci flow to uniformization and geometrization one needs to normalize the Ricci flow to obtain a flow which preserves volume. If one fails to do this, the problem is that for example instead of evolving a given three-dimensional manifold into one of Thurston's canonical forms, we might just shrink its size.

It is possible to construct a kind of moduli space of n-dimensional Riemannian manifolds, and then the Ricci flow really does give a geometric flow in the intuitive sense of particles flowing along flowlines in this moduli space. Hamilton showed that a compact Riemannian manifold always admits a short-time Ricci flow solution. Later Shi generalized the short-time existence result to complete manifolds of bounded curvature.

A fundamental problem in Ricci flow is to understand all the possible geometries of singularities.

When successful, this can lead to insights into the topology of manifolds. For instance, analyzing the geometry of singular regions that may develop in 3d Ricci flow, is the crucial ingredient in Perelman's proof the Poincare and Geometrization Conjectures.

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To study the formation of singularities it is useful, as in the study of other non-linear differential equations, to consider blow-ups limits. Intuitively speaking, one zooms into the singular region of the Ricci flow by rescaling time and space. Singularity models are ancient Ricci flows, i.

Understanding the possible singularity models in Ricci flow is an active research endeavor.

### Hamilton's Ricci Flow (Graduate Studies in Mathematics)

Then one considers the parabolically rescaled metrics. In the case that.

Otherwise the singularity is of Type II. It is known that the blow-up limits of Type I singularities are gradient shrinking Ricci solitons. In 3d the possible blow-up limits of Ricci flow singularities are well-understood.

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By Hamilton, Perelman and recent work by Brendle, blowing up at points of maximum curvature leads to one of the following three singularity models:. The first two singularity models arise from Type I singularities, whereas the last one arises from a Type II singularity.

In four dimensions very little is known about the possible singularities, other than that the possibilities are far more numerous than in three dimensions. To date the following singularity models are known. Note that the first three examples are generalizations of 3d singularity models.

The FIK shrinker models the collapse of an embedded sphere with self-intersection number To see why the evolution equation defining the Ricci flow is indeed a kind of nonlinear diffusion equation, we can consider the special case of real two-manifolds in more detail. Any metric tensor on a two-manifold can be written with respect to an exponential isothermal coordinate chart in the form.

These coordinates provide an example of a conformal coordinate chart, because angles, but not distances, are correctly represented. Take the coframe field. That is,. To compute the curvature tensor, we take the exterior derivative of the covector fields making up our coframe:. From these expressions, we can read off the only independent Spin connection one-form.

Take another exterior derivative. This is manifestly analogous to the best known of all diffusion equations, the heat equation. The reader may object that the heat equation is of course a linear partial differential equation —where is the promised nonlinearity in the p. The answer is that nonlinearity enters because the Laplace-Beltrami operator depends upon the same function p which we used to define the metric.

This computation suggests that, just as according to the heat equation an irregular temperature distribution in a hot plate tends to become more homogeneous over time, so too according to the Ricci flow an almost flat Riemannian manifold will tend to flatten out the same way that heat can be carried off "to infinity" in an infinite flat plate.

But if our hot plate is finite in size, and has no boundary where heat can be carried off, we can expect to homogenize the temperature, but clearly we cannot expect to reduce it to zero.

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