We establish an integral-geometric formula for minimal twospheres inside homogeneous three-spheres, and use it to provide a characterisation of each homogeneous metric on the three-dimensional real projective space as the unique metric with the largest possible twosystole among metrics with the same volume in its conformal class.
IntroductionLet RP 3 be the three-dimensional real projective space, and F denote the non-empty set consisting of all embedded surfaces in RP 3 that are diffeomorphic to the two-dimensional projective plane RP 2 . Given a Riemannian metric g on RP 3 , we defineIn this paper, the geometric invariant above will be called the two-systole of (RP 3 , g). The term has been used to name slightly different invariants in the literature, depending on the choice of the set F (cf. [10], Sections 1 and 4.A.7, and [5], Section 5). The first systematic study of such invariants was done by Berger in [5], where he sought generalisations of Pu's inequality [15] for the (one)-systole of real projective planes, i.e. the smallest length of a non-trivial loop in (RP 2 , g). Berger computed that the two-systole of the standard round metric g 1 on RP 3 , with constant sectional curvature one, is equal to 2π (see [5], Théorème 7.1). This number is precisely the area of the totally geodesic projective planes in (RP 3 , g 1 ).In [6], Bray, Brendle, Eichmair and Neves studied how the two-systole behaves under the Ricci flow, proving along the way a sharp upper bound for A(RP 3 , g) in terms of the minimum value of the scalar curvature of (RP 3 , g) (see [6], Theorems 1.1 and 1.2). An important part of their analysis was to show that the infimum defining A(RP 3 , g) is actually attained by an embedded area-minimising projective plane in (RP 3 , g) (see [6], Proposition 2.3).In this paper, we investigate how large can be the normalised two-systole, A(RP 3 , g) vol(RP 3 , g) 2 3, MSC classification codes: 53A10, 53C30.