# Croke isoperimetric inequality

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Let be a bounded domain in a complete Riemannian manifold with smooth boundary . A unit vector is said to be a direction of visibility at if the arc of the geodesic ray from up to the first boundary point is the shortest connection between the points and , i.e. . Let be the set of directions of visibility at and define the minimum visibility angle of by

where .

Then the following inequalities hold:

 (a1)
 (a2)

Both inequalities (a1) and (a2) are sharp in the sense that equality holds if and only if and is a hemi-sphere of a sphere of constant positive curvature.

In the proof of the second inequality, special versions of the Berger inequality and the Kazdan inequality are used.

#### References

 [a1] I. Chavel, "Riemannian geometry: A modern introduction" , Cambridge Univ. Press (1995) [a2] C.B. Croke, "Some isoperimetric inequalities and eigenvalue estimates" Ann. Sci. Ecole Norm. Sup. , 13 (1980) pp. 419–435
How to Cite This Entry:
Croke isoperimetric inequality. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Croke_isoperimetric_inequality&oldid=17819
This article was adapted from an original article by H. Kaul (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article