ON ISOSYSTOLIC INEQUALITIES FOR T n, RP n, AND M3 KEI NAKAMURA

Abstract. If a closed smooth n-manifold M admits a finite cover M ̂ whose Z/2Z-cohomology has the maximal cup-length, then for any riemannian metric g on M, we show that the systole Sys(M, g) and the volume Vol(M, g) of the riemannian manifold (M, g) are related by the following isosystolic inequality: Sys(M, g) n ≤ n!Vol(M, g). The inequality can be regarded as a generalization of Burago and Hebda’s inequality for closed essential surfaces and as a refinement of Guth’s inequality for closed n-manifolds whose Z/2Z-cohomology has the maximal cup-length. We also establish the same inequality in the context of possibly non-compact manifolds under a similar cohomological condition. The inequality applies to (i) T n and all other compact euclidean space forms, (ii) RPn and many other spherical space forms including the Poincare ́ dodecahedral space, and (iii) most closed essential 3-manifolds including all closed aspherical 3-manifolds. 1