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arXiv 0705.3827math.DGmath.APmath.GT
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宽度和中曲率流

Width and mean curvature flow

Tobias H. Colding, William P. Minicozzi

摘要:

给定同伦$n$ -球面上的一个黎曼度量,用一个连续的单参数闭曲线族将其扫过,这些曲线起始于点曲线,结束于点曲线。以连续的方式,尽可能地拉紧每个曲线,同时保持清料,从而拉紧清料。我们展示:在收紧的扫掠中,其长度接近于扫掠中最长曲线的长度的每条曲线本身必须接近于闭测地线。特别是,在扫掠中存在接近于闭测地线的曲线。利用扫掠法在2-球面上寻找闭测地线的工作可以追溯到1917年的伯克霍夫。作为一个应用,我们从上面的限制,由一个负常数,宽度的变化率的单参数家庭的凸超曲面,流动的中曲率。宽度是松散地说,直到一个常数的平方最短的闭曲线需要"拉过“$M$的长度。这种估计是精确的,并导致对消亡时的精确估计;参见。[CM1],[CM 2],其中对于由里奇流演化的同伦3-球,示出了二维宽度的变化率的类似界限(也参见Perelman)。

英文摘要:

Given a Riemannian metric on a homotopy $n$-sphere, sweep it out by a continuous one-parameter family of closed curves starting and ending at point curves. Pull the sweepout tight by, in a continuous way, pulling each curve as tight as possible yet preserving the sweepout. We show: Each curve in the tightened sweepout whose length is close to the length of the longest curve in the sweepout must itself be close to a closed geodesic. In particular, there are curves in the sweepout that are close to closed geodesics. Finding closed geodesics on the 2-sphere by using sweepouts goes back to Birkhoff in 1917. As an application, we bound from above, by a negative constant, the rate of change of the width for a one-parameter family of convex hypersurfaces that flows by mean curvature. The width is loosely speaking up to a constant the square of the length of the shortest closed curve needed to ``pull over'' $M$. This estimate is sharp and leads to a sharp estimate for the extinction time; cf. [CM1], [CM2] where a similar bound for the rate of change for the two dimensional width is shown for homotopy 3-spheres evolving by the Ricci flow (see also Perelman).

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