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A Crash Introduction to Gromov Hyperbolic Spaces

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A Crash Introduction to Gromov Hyperbolic Spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion A crash introduction to Gromov hyperbolic spaces Valentina Disarlo Universität Heidelberg Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion Geodesic metric spaces Definition A metric space (X; d) is proper if for every r > 0 the ball B(x; r) is compact. It is geodesic if every two points of X are joined by a geodesic. n R with the Euclidean distance dEucl. the infinite tree T with its length distance (every edge has length 1); x y w Figure: The infinite tree T Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion n Geodesic metric spaces: the hyperbolic space H n Disk model D n n 4 D ∶= {x ∈ R SSxS < 1} with the Riemannian metric induced by gx ∶= gEucl (1 − SSxSS2)2 n Upper half plane H n n 1 H ∶= {(x1;:::; xn) ∈ R S xn > 0} with the Riemannian metric induced by gx ∶= gEucl xn n ∀x; y ∈ D there exists a unique geodesic xy every geodesic segment can be extended indefinitely 2 2 Figure: radii/arcs ⊥ @D and half-circles/lines ⊥ @H Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion n Geodesic metric spaces: the hyperbolic space H n The boundary of H is defined as the space: n n @H = { geodesic rays c ∶ [0; ∞) → H }~ ∼ ′ ′ c ∼ c if and only if d(c(t); c (t)) < M n n−1 n n It can be topologized so that @H = S and Hn = H ∪ @H is compact. n Two geodesics can be: incident (1 common point in H , asymptotic (1 n common point on @H ), or ultraparallel (no common points). 2 Figure: @H =x -axis ∪{∞} Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion Motivation: Negative curvature without formulas n Riemannian manifold (M ; g) Ð→ intrinsic notion of curvature defined by g 2 (flat geometry) R has constant sectional curvature = 0 2 (spherical geometry) S has constant sectional curvature = 1 2 (hyperbolic geometry) H has constant sectional curvature = −1: geodesics diverge exponentially fast; geodesic triangles are thin... This talk: negative curvature for metric spaces using only geometric notions n analogies between trees and H Ð→ Gromov hyperbolic metric spaces No Riemannian geometry, no tensors, no painful formulas like this: 1 @ @ K = − Γ2 − Γ2 + Γ1 Γ2 − Γ1 Γ2 + Γ2 Γ2 − Γ2 Γ2 E @u 12 @v 11 12 11 11 12 12 12 11 22 (Gaussian curvature n = 2) Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion Geodesic metric spaces Let (X; dX) and (Y; dY ) be proper geodesic metric spaces. A map f ∶ X → Y is an isometry when dY (f (x); f (y)) = dX(x; y) for all x; y ∈ X; a (λ, )quasi-isometric embedding when dY (f (x); f (y)) ∼ dX(x; y): 1 d x; y d f x ; f y λd x; y for all x; y X ; λ X( ) − ≤ Y ( ( ) ( )) ≤ X( ) + ∈ a (λ, ) quasi-isometry when f is a (λ, )-q.isometric embedding and ∃C ≥ 0 such that every point of Y lies in a C-neighborhood of Imf . Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Gromov boundary Conclusion Quasi-isometries: Examples Every finite diameter space X is quasi-isometric to a point. All finite degree trees are quasi-isometric to each other. 2 R is not quasi-isometric to R. 2 2 R is not quasi-isometric to H A quasigeodesic is a quasi-isometric embedding γ ∶ I → X where I ⊂ R. Every geodesic in X is as quasi-geodesic 2 The curve t ↦ (t; log(1 + t)) is a quasi-geodesic in (R ; dE) but not a geodesic. Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity I : the Rips condition Let δ > 0. A geodesic triangle is called δ-slim if each of its sides is contained in the δ-neighborhood of the union of the other two sides. Definition (Criterion I) A space (X; d) is Gromov-hyperbolic (or δ-hyperbolic) if there exists a δ ≥ 0 such that every geodesic triangle is δ-slim. Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity I: Examples Gromov hyperbolic spaces are “thickened” versions of trees: n H (any complete Riemannian manifold with sectional curvature ≤ < 0). Trees are 0-hyperbolic: geodesic triangles are tripods x y w 2 R is not Gromov-hyperbolic Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity II: Gromov product Definition (Gromov Product) The Gromov product of x; y ∈ X with respect to w ∈ X is defined as 1 x; y w d x; w d y; w d x; y ( ) = 2 ( ( ) + ( ) − ( )) it measures the “defect” of triangle inequality: d(x; y) ≤ d(x; w) + d(y; w) (x; y)w measures how long the geodesics wx and wy travel together before diverging x y w Figure: In a tree (x; y)w is the distance from w to xy Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity II: Geometric interpretation (x; y)w measures how long wx and wy travel together before diverging There exists unique a; b; c ≥ 0 such that: d(x; w) = a + b d(x; y) = b + c d(y; w) = a + c The solutions are a = (x; y)w; b = (y; w)x and c = (x; w)y. y x c c x (x,y) w b a w w x b a w y y Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity II: the 4-point condition Definition (Criterion II: the 4-point condition) X is δ-hyperbolic if and only if for all x; y; z; w ∈ X we have: (x; y)w ≥ min((y; z)w; (x; z)w) − δ : x w y 2 Figure: (x; y)w ∼ d(w; [x; y]) in H Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Gromov hyperbolicity II: the 4-point condition Consider the tetrahedron T(x; y; w; z): x c y a a’ b b’ wc’ z ′ ′ ′ sum the lengths of the opposite sides a + a , b + b , c + c ; order the sums: S ≤ M ≤ L. The 4-point condition is equivalent to the following condition on T: L ≤ M + 2δ : Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces The Rips condition Gromov hyperbolicity The 4-point condition Gromov boundary Geodesics and quasi-geodesics Conclusion Geodesics and quasi-geodesics Definition (Hausdorff distance) Let A; B ⊂ X. The Hausdorff distance between A; B is defined by dH(A; B) = inf{ S A ⊆ N(B) and B ⊆ N(A)} Theorem (Stability of quasi-geodesics) Let X be δ-hyperbolic. For every λ ≥ 1 and ≥ 0 there exists Rλ, such that for every (λ, )-quasigeodesic c ∶ [a; b] → X is R-close to every geodesic segment [c(a); c(b)] we have: dH(Im c; [c(a); c(b)]) ≤ Rλ, : c R c(a) c(b) Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Definition Gromov boundary Bonus: visual metric on @X Conclusion @X Gromov boundary: asymptotic geodesic rays Definition (Gromov-boundary ) We define the Gromov boundary of X as @r;wX ∶= {c ∶ [0; ∞) → X geodesic ray ; c(0) = w}~ ∼ ′ ′ where c ∼ c if and only if dH(Im c; Im c ) < ∞. A basis of neighborhoods for p ∈ @r;wX is given by ′ ′ V p; R q @X c p ; c q with lim inf c s ; c t w R : ( ) = { ∈ S∃ ∈ [ ] ∈ [ ] s;t→∞ ( ( ) ( )) ≥ } (Visibility) For every x ≠ y ∈ @X there exists a bi-infinite geodesic connecting them Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Definition Gromov boundary Bonus: visual metric on @X Conclusion Properties of @X A few facts (theorems): @r;wX does not depend on w; @X admits a metrizable topology and X ∶= X ∪ @X is compact. Theorem ′ ′ Let X; X be hyperbolic spaces. If f ∶ X → X is a q.i.-embedding then the map ′ f@ ∶ @X → @X [c] ↦ [f ○ c] is a topological embedding. If f is a quasi-isometry then f@ is a homeomorphism. Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Definition Gromov boundary Bonus: visual metric on @X Conclusion Gromov-boundaries: Hall of Fame The Gromov boundary of the tree Tn is a Cantor set. n n n−1 The Gromov boundary of H is @H = S . Corollary n m H and H are quasi-isometric if and only if n = m. Valentina Disarlo A crash introduction to Gromov hyperbolic spaces Language of metric spaces Gromov hyperbolicity Definition Gromov boundary Bonus: visual metric on @X Conclusion @X Gromov boundary: converging sequences Fix w ∈ X.
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