Thursday, 16 April 2009

riemannian geometry - Injectivity radius and the cut locus

Consider a connected, complete and compact Riemannian manifold M. Is it correct that the following equality holds: textinj(x)=textdistleft(x,textCuL(x)right)? Or in words that the injectivity radius of a point is the distance from the point to its cut locus.



Here is my explanation: As the manifold is compact and complete, then the cut locus textCuL(x) is compact as well[1]. Thus, there exists a point yintextCuL(x) such that textdistleft(x,textCuL(x)right)=textdist(x,y). Since y is a cut point of x, there exists a tangent vector xi0inTxM such that y=expxleft(c(xi0)xi0right)[2], where c(xi0) is the distance from x to its cut point in the xi0 direction. This in turn means that textdist(x,y)=c(xi0).



Recall that textinj(x)=infxiinTxM(c(xi)). This means that textinj(x)leqc(xi0)=textdist(x,y)=textdistleft(x,textCuL(x)right). If textinj(x)<c(xi0), then since M is compact, it means that there exists some other tangent vector xiinTxM with c(xi)<c(xi0). But this means that expx(c(xi)xi) is a cut point of x closer to it then y, and this is a contradiction.




[1] See Contributions to Riemannian Geometry in the Large by W. Klingenberg



[2] Here I'm using the notation of I. Chavel in his book Riemannian Geometry - Modern Introduction.




Update(@dror)
Today I finally found a copy of the book *Riemannian Geometry" by Takashi Sakai, and there the above is stated as proposition 4.13 in chapter 3.
Thanks anyway.

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