Abstract
We present a new distance characterization of Aleksandrov spaces of non-positive curvature. By introducing a quasilinearization for abstract metric spaces we draw an analogy between characterization of Aleksandrov spaces and inner product spaces; the quasi-inner product is defined by means of the quadrilateral cosine—a metric substitute for the angular measure between two directions at different points. Our main result states that a geodesically connected metric space \({\left(\mathcal{M},\rho\right)}\) is an Aleksandrov \({\Re_{0}}\) domain (also known as a CAT(0) space) if and only if the quadrilateral cosine does not exceed one for every two pairs of distinct points in \({\mathcal{M}}\) . We also observe that a geodesically connected metric space \({\left(\mathcal{M},\rho\right)}\) is an \({\Re_{0}}\) domain if and only if, for every quadruple of points in \({\mathcal{M}}\) , the quadrilateral inequality (known as Euler’s inequality in \({\mathbb{R}^{2}}\)) holds. As a corollary of our main result we give necessary and sufficient conditions for a semimetric space to be an \({\Re_{0}}\) domain. Our results provide a complete solution to the Curvature Problem posed by Gromov in the context of metric spaces of non-positive curvature.
Similar content being viewed by others
References
Aleksandrov, A.D.: A theorem on triangles in a metric space and some of its applications. In: Trudy Mat. Inst. Steklov. 38, 5–23. Trudy Mat. Inst. Steklov. 38, Izdat. Akad. Nauk SSSR, Moscow (1951) (in Russian)
Alexandrow, A.D.: Über eine Verallgemeinerung der Riemannschen Geometrie. Schr. Forschungsinst. Math. 1, 33–84 (1957)
Aleksandrov, A.D.: Ruled surfaces in metric spaces. Vestnik Leningrad. Univ. 12(1), 5–26, 207 (1957) (in Russian)
Amir, D.: Characterizations of inner product spaces. Séminaire d’Analyse Fonctionelle 1984/1985, pp. 77–93. Publ. Math. Univ. Paris VII, 26, Univ. Paris VII, Paris (1986)
Berg, I.D., Nikolaev, I.G.: On a distance between directions in an Aleksandrov space of curvature ≤ K. Michigan Math. J. 45(2), 257–289 (1998)
Berg, I.D., Nikolaev, I.G.: On an extremal property of quadrilaterals in an Aleksandrov space of curvature ≤ K. In: The interaction of analysis and geometry, pp. 1–16, Contemp. Math., vol. 424. Am. Math. Soc., Providence, RI (2007)
Berg, I.D., Nikolaev, I.G.: On a distance characterization of A.D. Aleksandrov spaces of non-positive curvature. Dokl. Akad. Nauk, 414(1), 1–3 (2007) (In Russian) English translation: Dokl. Math., 75(3), 336–338 (2007)
Berestovskii, V.N.: Spaces with bounded curvature and distance geometry. Sibirsk. Mat. Zh. 27(1), 11–25, 197 (1986) (In Russian), English translation: Sib. Mat. J. 27, 8–19 (1986)
Berestovskii, V.N., Nikolaev, I.G.: Multidimensional generalized Riemannian spaces. Geometry IV, 165–243, 245–250. In: Encyclopaedia Math. Sci., vol. 70, Springer, Berlin (1993)
Blumenthal, L.M.: Theory and Applications of Distance Geometry, 2nd edn. Chelsea Publishing Co., New York (1970)
Bruhat, F., Tits, J.: Groupes réductifs sur un corps local. Inst. Hautes Études Sci. Publ. Math. 41, 5–251 (1972)
Day, M.M.: Some characterizations of inner-product spaces. Trans. Am. Math. Soc. 62, 320–337 (1947)
Day, M.M.: Normed linear spaces. Ergebnisse der Mathematik und ihrer Grenzgebiete. Neue Folge, 21 . Reelle Funktionen Springer-Verlag, Berlin-Göttingen-Heidelberg, Reihe (1958)
Enflo, P.: On the nonexistence of uniform homeomorphisms between L p -spaces. Ark. Mat. 8, 103–105 (1969)
Enflo, P.: Uniform structures and square roots in topological groups. I. Israel J. Math. 8, 230–252 (1970)
Enflo, P.: Uniform structures and square roots in topological groups. II. Israel J. Math. 8, 253–272 (1970)
Euler, L.: Variae demonstrationes geometriae. Novi Commentarii academiae scientiarum Petropolitanae 1, 49–66 (1750) (Opera Omnia, Ser. 1, 26, 29–32 (1953))
Foertsch, T., Lytchak, A., Schroeder, V.: Nonpositive curvature and the Ptolemy inequality. International Mathematics Research Notices (2007), article ID rnm100, 15 p., doi:10.1093/imrn/rnm100
Gromov, M.: Metric Structures for Riemannian and Non-Riemannian Spaces. Based on the 1981 French original. With appendices by M. Katz, P. Pansu and S. Semmes. Translated from the French by Sean Michael Bates. Progress in Mathematics, vol. 152. Birkhäuser Boston, Inc., Boston, MA (1999)
Jordan, P., Von Neumann, J.: On inner products in linear, metric spaces. Ann. of Math., 2nd Ser. 36(3), 719–723 (1935)
Kay, D.C.: The ptolemaic inequality in Hilbert geometries. Pacific J. Math. 21, 293–301 (1967)
Korevaar, N.J., Schoen, R.M.: Sobolev spaces and harmonic maps for metric space targets. Comm. Anal. Geom. 1(4), 561–659 (1993)
Lafont, J.-.F, Prassidis, S.: Roundness properties of groups. Geom. Dedicata 117, 137–160 (2006)
Nikolaev, I.G.: Axioms of Riemannian geometry. Dokl. Akad. Nauk SSSR 307(4), 812–814 (1989) (In Russian); English translation: Soviet Math. Dokl. 40(1), 172–174 (1990)
Nikolaev, I.G.: A metric characterization of Riemannian spaces. Siberian Adv. Math. 9(4), 1–58 (1999)
Reshetnyak, Yu.G.: Non-expanding mappings in a space of curvature not greater than K. Sibirsk. Mat. Zh. s9, 918–927 (1968) (in Russian); English translation: Sib. Math. J. 9, 683–689 (1968)
Schoenberg, I.J.: A remark on M. M. Day’s characterization of inner-product spaces and a conjecture of L. M. Blumenthal. Proc. Am. Math. Soc. 3, 961–964 (1952)
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
About this article
Cite this article
Berg, I.D., Nikolaev, I.G. Quasilinearization and curvature of Aleksandrov spaces. Geom Dedicata 133, 195–218 (2008). https://doi.org/10.1007/s10711-008-9243-3
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s10711-008-9243-3