Gromov's systolic inequality for essential manifolds
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In Riemannian geometry, M. Gromov's systolic inequality for essential n-manifolds M dates from 1983. It is a lower bound for the volume of an arbitrary metric on M, in terms of its homotopy 1-systole. The homotopy 1-systole is the least length of a non-contractible loop. We will denote the homotopy 1-systole by the symbol 
, so as to avoid confusion with the homology 1-systole. Then Gromov's inequality takes the form
where Cn is a universal constant only depending on the dimension of M. Thus Gromov's systolic inequality for essential manifolds can be viewed as a generalisation, albeit non-optimal, of Loewner's torus inequality and Pu's inequality for the real projective plane.
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[edit] Essential manifolds
A closed manifold is called essential if its fundamental class defines a nonzero element in the homology of its fundamental group, or more precisely in the homology of the corresponding Eilenberg-MacLane space. Here the fundamental class is taken in homology with integer coefficients if the manifold is orientable, and in coefficients modulo 2, otherwise.
Examples of essential manifolds include aspherical manifolds, real projective spaces, and lens spaces.
[edit] Proofs of Gromov's inequality
Gromov's original 1983 proof is about 35 pages long. It relies on a number of techniques and inequalities of global Riemannian geometry. The starting point of the proof is the imbedding of X into the Banach space of Borel functions on X, equipped with the sup norm. The imbedding is defined by mapping a point p of X, to the real function on X given by the distance from the point p. The proof utilizes the coarea inequality, the isoperimetric inequality, the cone inequality, and the deformation theorem of Herbert Federer.
[edit] Filling invariants and recent work
One of the key ideas of the proof is the introduction of filling invariants, namely the filling radius and the Filling Volume of X. Namely, Gromov proved a sharp inequality relating the systole and the filling radius,
valid for all essential manifolds X; as well as an inequality
valid for all closed manifolds X.
It was shown by M. Brunnbauer recently that the filling invariants, unlike the systolic invariants, are independent of the topology of the manifold in a suitable sense.
Recently, alternative proofs of Gromov's systolic inequality for essential manifolds have been found by S. Wenger and independently by L. Guth.
[edit] See also
- Filling area conjecture
- Gromov's inequality for complex projective space
- Loewner's torus inequality
- Pu's inequality for the real projective plane
- Systolic geometry
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