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I call a function f defined and valued on a domain A in the plane convex if it maps convex areas to convex areas. Some obvious example of convex functions. If f is also a bijection, what can we say more about it? I guessed that if f is a diffeomorphism(C2) of the complex plane then it is linear, say az+b. This is related to a quadratic form of its differentials fxx,fyy,fxy. However, I cannot strictly confirm this. Is there any classical analysis addressing with this question? If any, please help show the sources.

edited Mar 20 2012 at 17:41

asked Mar 20 2012 at 17:08

Zhipeng Lu
57●5

alvarezpaiva · Accepted Answer · 2012-03-20 17:33:38Z UTC

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I think that A bijection from the plane to itself that sends convex sets into convex sets is an affine transformation.

Indeed, it must not be hard to see that such bijection sends lines to lines (into what would it send a half-plane given that it is convex and has a convex complement??) and then one applies the fundamental theorem of affine geometry.

answered Mar 20 2012 at 17:33

alvarezpaiva
4,132●5●21

So this quesion is indeed trivial. Thanks for your tips. – Zhipeng Lu Mar 20 2012 at 17:45

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nice! it seems it's a rule: when dealing with some problem about convex objects, never forget to start by the linear or affine ones... – Pietro Majer Mar 20 2012 at 17:54

Well, I think that there are little details to be checked, but if you assume the map is a homeomorphism, then its easy. – alvarezpaiva Mar 20 2012 at 20:10

complex convex functions

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