By Hans Grauert
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Extra resources for Complex Analysis and Algebraic Geometry: Proceedings of a Conference, Held in Gottingen, June 25 - July 2, 1985
10* The earlier work on ancient solutions and all that can be found in [H 4, §16 − 22, 25, 26]. 1 Let φ be a decreasing function of one variable, tending to zero at infinity. A solution to the Ricci flow is said to have φ-almost nonnegative curvature if it satisfies Rm(x, t) ≥ −φ(R(x, t))R(x, t) for each (x, t). Theorem. Given ǫ > 0, κ > 0 and a function φ as above, one can find r0 > 0 with the following property. 1. Proof. An argument by contradiction. Take a sequence of r0 converging to zero, and consider the solutions gij (t), such that the conclusion does not hold for some (x0 , t0 ); moreover, by tampering with the condition t0 ≥ 1 a little bit, choose among all such (x0 , t0 ), in the solution under consideration, the one with nearly the smallest curvature Q.
Slightly abusing notation, we’ll drop the indices α, β when we consider an individual solution. Let t¯ be the first time when the assumption is violated at some point x¯; clearly such time exists, because it is an open condition. 2 we have uniform κ-noncollapsing on [0, t¯]. 2 are also valid on [0, t¯]; moreover, since h << r, it follows from Claim 1 that the solution is defined on the whole parabolic neighborhood indicated there in case R(x0 , t0 ) ≤ r−2 . Scale our solution about (¯ x, t¯) with factor R(¯ x, t¯) ≥ r−2 and take a limit for ¯ subsequences of α, β → ∞.
The point is to make h arbitrarily small while keeping r bounded away from zero. Notation and terminology B(x, t, r) denotes the open metric ball of radius r, with respect to the metric at time t, centered at x. P (x, t, r, △t) denotes a parabolic neighborhood, that is the set of all points (x′ , t′ ) with x′ ∈ B(x, t, r) and t′ ∈ [t, t + △t] or t′ ∈ [t + △t, t], depending on the sign of △t. A ball B(x, t, ǫ−1 r) is called an ǫ-neck, if, after scaling the metric with factor −2 r , it is ǫ-close to the standard neck S2 × I, with the product metric, where S2 has constant scalar curvature one, and I has length 2ǫ−1 ; here ǫ-close refers to C N topology, with N > ǫ−1 .