Filling area conjecture

In Riemannian geometry, an area of mathematics, Mikhail Gromov's filling area conjecture asserts that among all possible fillings of the Riemannian circle of length 2 $π$ by a surface with the strongly isometric property, the round hemisphere has the least area. Here the Riemannian circle refers to the unique closed 1-dimensional Riemannian manifold of total 1-volume 2 $π$ and Riemannian diameter $π$ .

Explanation

To explain the conjecture, we start with the observation that the equatorial circle of the unit 2-sphere

S^{2}\subset \mathbb {R} ^{3}\,\!

is a Riemannian circle S¹ of length 2 $π$ and diameter $π$ . More precisely, the Riemannian distance function of S¹ is the restriction of the ambient Riemannian distance on the sphere. This property is not satisfied by the standard imbedding of the unit circle in the Euclidean plane, where a pair of opposite points are at distance 2, not $π$ .

We consider all fillings of S¹ by a surface, such that the restricted metric defined by the inclusion of the circle as the boundary of the surface is the Riemannian metric of a circle of length 2 $π$ . The inclusion of the circle as the boundary is then called a strongly isometric imbedding of the circle. In 1983 Gromov conjectured that the round hemisphere gives the "best" way of filling the circle among all filling surfaces.

Relation to Pu's inequality

An animation of the Roman surface representing RP² in R³

The case of simply-connected fillings is equivalent to Pu's inequality for the real projective plane RP². Recently the case of genus-1 fillings was settled affirmatively, as well (see Bangert et al). Namely, it turns out that one can exploit a half-century old formula by J. Hersch from integral geometry. Namely, consider the family of figure-8 loops on a football, with the self-intersection point at the equator (see figure at the beginning of the article). Hersch's formula expresses the area of a metric in the conformal class of the football, as an average of the energies of the figure-8 loops from the family. An application of Hersch's formula to the hyperelliptic quotient of the Riemann surface proves the filling area conjecture in this case.

References

Bangert, V.; Croke, C.; Ivanov, S.; Katz, M.: Filling area conjecture and ovalless real hyperelliptic surfaces, Geometric and Functional Analysis (GAFA) 15 (2005), no. 3, 577–597. See arXiv:math.DG/0405583
Gromov, M., "Filling Riemannian manifolds". J. Diff. Geom. 18 (1983), pp. 1–147.
Katz, Mikhail G. (2007), Systolic geometry and topology, Mathematical Surveys and Monographs, 137, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-4177-8

Systolic geometry

1-systoles of surfaces	Loewner's torus inequality Pu's inequality Filling area conjecture Bolza surface Systoles of surfaces Eisenstein integers

1-systoles of manifolds	Gromov's systolic inequality for essential manifolds Essential manifold Filling radius Hermite constant

Higher systoles	Gromov's inequality for complex projective space Systolic freedom Systolic category

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Filling area conjecture

Explanation

Relation to Pu's inequality

See also

References