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ISODIAMETRIC SETS IN THE HEISENBERG GROUP

28 pages
ISODIAMETRIC SETS IN THE HEISENBERG GROUP G.P. LEONARDI, S. RIGOT, AND D. VITTONE Abstract. In the sub-Riemannian Heisenberg group equipped with its Carnot-Caratheodory metric and with a Haar measure, we consider iso- diametric sets, i.e. sets maximizing the measure among all sets with a given diameter. In particular, given an isodiametric set, and up to negli- gible sets, we prove that its boundary is given by the graphs of two locally Lipschitz functions. Moreover, in the restricted class of rotationally in- variant sets, we give a quite complete characterization of any compact (rotationally invariant) isodiametric set. More specifically, its Steiner symmetrization with respect to the Cn-plane is shown to coincide with the Euclidean convex hull of a CC-ball. At the same time, we also prove quite unexpected non-uniqueness results. 1. Introduction The classical isodiametric inequality in the Euclidean space says that balls maximize the volume among all sets with a given diameter. This was origi- nally proved by Bieberbach [5] in R2 and by Urysohn [14] in Rn, see also [6]. In this paper we are interested in the case of the Heisenberg group Hn equipped with its Carnot-Caratheodory distance d and with the Haar mea- sure L2n+1 (see Section 2 for the definitions).

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ISODIAMETRIC SETS IN THE HEISENBERG GROUP
G.P. LEONARDI, S. RIGOT, AND D. VITTONE
Abstract.In the sub-Riemannian Heisenberg group equipped with its Carnot-Carath´eodorymetricandwithaHaarmeasure,weconsideriso-diametric sets, i.e. sets maximizing the measure among all sets with a given diameter. In particular, given an isodiametric set, and up to negli-gible sets, we prove that its boundary is given by the graphs of two locally Lipschitz functions. Moreover, in the restricted class ofrotationally in-variant sets, we give a quite complete characterization of any compact (rotationally invariant) isodiametric set. More specifically, its Steiner symmetrization with respect to theCn-plane is shown to coincide with the Euclidean convex hull of a CC-ball. At the same time, we also prove quite unexpected non-uniqueness results.
1.onnItroducti
The classical isodiametric inequality in the Euclidean space says that balls maximize the volume among all sets with a given diameter. This was origi-nally proved by Bieberbach [5] inR2and by Urysohn [14] inRn, see also [6]. In this paper we are interested in the case of the Heisenberg groupHn equippedwithitsCarnot-Carathe´odorydistancedand with the Haar mea-sureL2n+1(see Section 2 for the definitions). aim is to study Ourisodiamet-ric setsmeasure among sets with a given diameter., i.e. maximizing the  sets
Recalling that the homogeneous dimension ofHnis 2n we define the+ 2, isodiametric constantCIby CI= supL2n+1(F)/(diamF)2n+2
2000Mathematics Subject Classification.53C17, 28A75, 49Q15, 22E30. Key words and phrases.Isodiametric problem, Heisenberg group. The first and third authors have been supported by E.C. project “GALA”, MIUR, GNAMPA project “Metodi geometrici per analisi in spazi non Euclidei: spazi metrici doubling, gruppi di Carnot e spazi di Wiener” (2009) and, respectively, by the University of Modena and Reggio Emilia and the University of Padova, Italy. The second author wishes to thank the Department of Pure and Applied Mathematics, University of Modena and Reggio Emilia, where part of the work was done, and also the GNAMPA project for financial support. The first and third authors are pleased to thank theLaboratoireJ.-A.Dieudonn´e,Universit´edeNiceSophia-Antipolis,forthehospitality during the completion of a first draft of the paper. 1
2 G.P. LEONARDI, S. RIGOT, AND D. VITTONE where the supremum is taken among all setsFHnwith positive and finite diameter. Sets realizing the supremum do exist, see [12] or Theorem 3.1 below. Since the closure of any such set is a compact set that still realizes the supremum, we consider the classIof compact isodiametric sets, I={EHn;Ecompact,diamE >0,L2n+1(E) =CI(diamE)2n+2}. In other words, due to the presence of dilations inHn,Idenotes the class of compact sets that maximize theL2n+1-measure among all sets with the same diameter. In contrast to the Euclidean case, balls in (Hn, d) are not isodiametric (see [12]) and we shall give in this paper some further and refined evidence that the situation is indeed quite different from the Euclidean one. Before describing our main results let us recall some classical motivations and consequences coming from the study of isodiametric type problems. First the isodiametric constantCIcoincides with the ratio between the mea-sureL2n+1and the (2n+2)-dimensional Hausdorff measureH2n+2in (Hn, d), namely, L2n+1=CIH2n+2, whereH2n+2(A) = limδ0inf{Pi(diamAi)2n+2;A⊂ ∪iAi,diamAiδ}. This can actually be generalized to any Carnot group equipped with a ho-mogeneous distance (see [12]), and for abelian Carnot groups one recovers the well-known Euclidean situation. We also refer the interested reader to [1] where some relationships between different intrinsic volumes that can be defined in sub-Riemannian geometry are studied. As a consequence, the knowledge of the numerical value of the isodiametric constantCI, or equivalently the explicit description of isodiametric sets, gives non trivial information about the geometry of the metric space (Hnd) , and about the measureH2n+2which may be considered as a natural measure from the metric point of view. There are also some links with the Besicovitch 1/2-problem which is in turn related to the study of the connections between densities and rectifi-ability. Let us only briefly sketch here this connection. We refer to [11] for a more detailed introduction and known results about the Besicovitch 1/2-problem and [12] for more details about the connection between the iso-diametric problem in Carnot groups and the Besicovitch 1/2-problem. Let σn(M, d) denote the density constant of the metric space (M, d). It is the smallest number such that every subset with finiteHn-measure havingn-dimensional lower density strictly greater thanσn(M, d) atHn-almost all of its points isn-rectifiable (see [11] for the precise definition). The validity of the boundσn(M, d)1/2 for any separable metric space (M, d), which was conjectured long ago by A.S. Besicovitch for the one-dimensional density constant inR2(see [4]), is known as the generalized Besicovitch 1/2-problem.