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International Congress of Mathematicians

International Congress of Mathematicians

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ICM 2002 • Vol. II • 221-230Positively Curved Surfacesin the Three-sphereB. Andrews*AbstractIn this talk I will discuss an example <strong>of</strong> the use <strong>of</strong> fully nonlinear parabolicflows to prove geometric results. I will emphasise the fact that there is a widevariety <strong>of</strong> geometric parabolic equations to choose from, and to get the bestresults it can be very important to choose the best flow. I will illustrate thisin the setting <strong>of</strong> surfaces in a three-dimensional sphere.There are quite a few relevant results for surfaces in the sphere satisfyingvarious kinds <strong>of</strong> curvature equations, including totally umbillic surfaces,minimal surfaces and constant mean curvature surfaces, and intrinsically flatsurfaces. Parabolic flows can strengthen such results by allowing classes <strong>of</strong>surfaces satisfying curvature inequalities rather than equalities: This was firstdone by Huisken, who used mean curvature flow to deform certain classes <strong>of</strong>surfaces to totally umbillic surfaces. This motivates the question "What is theoptimal result <strong>of</strong> this kind?" — that is, what is the weakest pointwise curvaturecondition which defines a class <strong>of</strong> surfaces which retracts to the space <strong>of</strong>great spheres?The answer to this question can be guessed in view <strong>of</strong> the examples. Toprove it requires a surprising choice <strong>of</strong> evolution equation, forced by the requirementthat the pointwise curvature condition be preserved.I will conclude by mentioning some other geometric situations in whichstrong results can be proved by choosing the best possible evolution equation.2000 Mathematics Subject Classification: 53C44, 53C40.Keywords and Phrases: Surfaces, Curvature, Parabolic equations.1. IntroductionMy aim in this talk is to demonstrate the use <strong>of</strong> fully nonlinear parabolic evolutionequations as tools for proving results in differential geometry. I will emphasisethe fact that there is a wide variety <strong>of</strong> flows which are geometrically defined and* Centre for Mathematics and its Applications, Australian National University, ACT 0200,Australia. E-mail: andrews@maths.anu.edu.au

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