Radius of convexity

This is a glossary of some terms used in Riemannian geometry and metric geometry — it doesn't cover the terminology of differential topology.

The following articles may also be useful; they either contain specialised vocabulary or provide more detailed expositions of the definitions given below.

* Connection
* Curvature
* Metric space
* Riemannian manifold
See also:
* Glossary of general topology
* Glossary of differential geometry and topology
* List of differential geometry topics

Unless stated otherwise, letters ''X'', ''Y'', ''Z'' below denote metric spaces, ''M'', ''N'' denote Riemannian manifolds, , ''xy'', or $, xy, _X$ denotes the distance between points ''x'' and ''y'' in ''X''. Italic ''word'' denotes a self-reference to this glossary.

''A caveat'': many terms in Riemannian and metric geometry, such as ''convex function'', ''convex set'' and others, do not have exactly the same meaning as in general mathematical usage.

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A

Alexandrov space a generalization of Riemannian manifolds with upper, lower or integral curvature bounds (the last one works only in dimension 2)

Almost flat manifold

Arc-wise isometry the same as ''path isometry''.

Autoparallel the same as ''totally geodesic''

B

Barycenter, see ''center of mass''.

bi-Lipschitz map. A map $f:X\to Y$ is called bi-Lipschitz if there are positive constants ''c'' and ''C'' such that for any ''x'' and ''y'' in ''X''
:$c, xy, _X\le, f(x)f(y), _Y\le C, xy, _X$

Busemann function given a '' ray'', γ : \gamma(t)-p, -t)

C

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Cartan–Hadamard theorem is the statement that a connected, simply connected complete Riemannian manifold with non-positive sectional curvature is diffeomorphic to Rⁿ via the exponential map; for metric spaces, the statement that a connected, simply connected complete geodesic metric space with non-positive curvature in the sense of Alexandrov is a (globally) CAT(0) space.

Élie Cartan, Cartan extended Einstein's General relativity to Einstein–Cartan theory, using Riemannian-Cartan geometry instead of Riemannian geometry. This extension provides affine torsion, which allows for non-symmetric curvature tensors and the incorporation of spin–orbit coupling.

Center of mass. A point ''q'' ∈ ''M'' is called the center of mass of the points $p_1,p_2,\dots,p_k$ if it is a point of global minimum of the function

:$f(x)=\sum_i , p_ix, ^2$

Such a point is unique if all distances $, p_ip_j,$ are less than ''radius of convexity''.

Christoffel symbol

Collapsing manifold

Complete space

Completion

Conformal map is a map which preserves angles.

Conformally flat a manifold ''M'' is conformally flat if it is locally conformally equivalent to a Euclidean space, for example standard sphere is conformally flat.

Conjugate points two points ''p'' and ''q'' on a geodesic $\gamma$ are called conjugate if there is a Jacobi field on $\gamma$ which has a zero at ''p'' and ''q''.

Convex function. A function ''f'' on a Riemannian manifold is a convex if for any geodesic $\gamma$ the function $f\circ\gamma$ is convex. A function ''f'' is called $\lambda$-convex if for any geodesic $\gamma$ with natural parameter $t$, the function $f\circ\gamma(t)-\lambda t^2$ is convex.

Convex A subset ''K'' of a Riemannian manifold ''M'' is called convex if for any two points in ''K'' there is a ''shortest path'' connecting them which lies entirely in ''K'', see also ''totally convex''.

Cotangent bundle

Covariant derivative

Cut locus

D

Diameter of a metric space is the supremum of distances between pairs of points.

Developable surface is a surface isometric to the plane.

Dilation of a map between metric spaces is the infimum of numbers ''L'' such that the given map is ''L''- Lipschitz.

E

Exponential map: Exponential map (Lie theory), Exponential map (Riemannian geometry)

F

Finsler metric

First fundamental form for an embedding or immersion is the pullback of the metric tensor.

G

Geodesic is a curve which locally minimizes distance.

Geodesic flow is a flow on a tangent bundle ''TM'' of a manifold ''M'', generated by a vector field whose trajectories are of the form $(\gamma(t),\gamma'(t))$ where $\gamma$ is a geodesic.

Gromov-Hausdorff convergence

Geodesic metric space is a metric space where any two points are the endpoints of a minimizing geodesic.

H

Hadamard space is a complete simply connected space with nonpositive curvature.

Horosphere a level set of ''Busemann function''.

I

Injectivity radius The injectivity radius at a point ''p'' of a Riemannian manifold is the largest radius for which the exponential map at ''p'' is a diffeomorphism. The injectivity radius of a Riemannian manifold is the infimum of the injectivity radii at all points. See also cut locus.

For complete manifolds, if the injectivity radius at ''p'' is a finite number ''r'', then either there is a geodesic of length 2''r'' which starts and ends at ''p'' or there is a point ''q'' conjugate to ''p'' (see conjugate point above) and on the distance ''r'' from ''p''. For a closed Riemannian manifold the injectivity radius is either half the minimal length of a closed geodesic or the minimal distance between conjugate points on a geodesic.

Infranilmanifold Given a simply connected nilpotent Lie group ''N'' acting on itself by left multiplication and a finite group of automorphisms ''F'' of ''N'' one can define an action of the semidirect product $N \rtimes F$ on ''N''. An orbit space of ''N'' by a discrete subgroup of $N \rtimes F$ which acts freely on ''N'' is called an ''infranilmanifold''. An infranilmanifold is finitely covered by a nilmanifold.

Isometry is a map which preserves distances.

Intrinsic metric

J

Jacobi field A Jacobi field is a vector field on a geodesic γ which can be obtained on the following way: Take a smooth one parameter family of geodesics $\gamma_\tau$ with $\gamma_0=\gamma$, then the Jacobi field is described by
:$J(t)=\left. \frac \_.$

Jordan curve

K

Killing vector field

L

Length metric the same as ''intrinsic metric''.

Levi-Civita connection is a natural way to differentiate vector fields on Riemannian manifolds.

Lipschitz convergence the convergence defined by Lipschitz metric.

Lipschitz distance between metric spaces is the infimum of numbers ''r'' such that there is a bijective ''bi-Lipschitz'' map between these spaces with constants exp(-''r''), exp(''r'').

Lipschitz map

Logarithmic map is a right inverse of Exponential map.

M

Mean curvature

Metric ball

Metric tensor

Minimal surface is a submanifold with (vector of) mean curvature zero.

N

Natural parametrization is the parametrization by length.

Net. A subset ''S'' of a metric space ''X'' is called $\epsilon$-net if for any point in ''X'' there is a point in ''S'' on the distance $\le\epsilon$. This is distinct from topological nets which generalize limits.

Nilmanifold: An element of the minimal set of manifolds which includes a point, and has the following property: any oriented $S^1$-bundle over a nilmanifold is a nilmanifold. It also can be defined as a factor of a connected nilpotent Lie group by a lattice.

Normal bundle: associated to an imbedding of a manifold ''M'' into an ambient Euclidean space $^N$, the normal bundle is a vector bundle whose fiber at each point ''p'' is the orthogonal complement (in $^N$) of the tangent space $T_pM$.

Nonexpanding map same as ''short map''

P

Parallel transport

Polyhedral space a simplicial complex with a metric such that each simplex with induced metric is isometric to a simplex in Euclidean space.

Principal curvature is the maximum and minimum normal curvatures at a point on a surface.

Principal direction is the direction of the principal curvatures.

Path isometry

Proper metric space is a metric space in which every closed ball is compact. Equivalently, if every closed bounded subset is compact. Every proper metric space is complete.

Q

Quasigeodesic has two meanings; here we give the most common. A map $f: I \to Y$ (where $I\subseteq \mathbb R$ is a subsegment) is called a ''quasigeodesic'' if there are constants $K \ge 1$ and $C \ge 0$ such that for every $x,y\in I$
:$d(x,y)-C\le d(f(x),f(y))\le Kd(x,y)+C.$
Note that a quasigeodesic is not necessarily a continuous curve.

Quasi-isometry. A map $f:X\to Y$ is called a ''quasi-isometry'' if there are constants $K \ge 1$ and $C \ge 0$ such that
:$d(x,y)-C\le d(f(x),f(y))\le Kd(x,y)+C.$
and every point in ''Y'' has distance at most ''C'' from some point of ''f''(''X'').
Note that a quasi-isometry is not assumed to be continuous. For example, any map between compact metric spaces is a quasi isometry. If there exists a quasi-isometry from X to Y, then X and Y are said to be quasi-isometric.

R

Radius of metric space is the infimum of radii of metric balls which contain the space completely.

Radius of convexity at a point ''p'' of a Riemannian manifold is the largest radius of a ball which is a ''convex'' subset.

Ray is a one side infinite geodesic which is minimizing on each interval

Riemann curvature tensor

Riemannian manifold

Riemannian submersion is a map between Riemannian manifolds which is submersion and ''submetry'' at the same time.

S

Second fundamental form is a quadratic form on the tangent space of hypersurface, usually denoted by II, an equivalent way to describe the ''shape operator'' of a hypersurface,
:$\text(v,w)=\langle S(v),w\rangle$
It can be also generalized to arbitrary codimension, in which case it is a quadratic form with values in the normal space.

Shape operator for a hypersurface ''M'' is a linear operator on tangent spaces, ''S''_''p'': ''T''_''p''''M''→''T''_''p''''M''. If ''n'' is a unit normal field to ''M'' and ''v'' is a tangent vector then
:$S(v)=\pm \nabla_n$
(there is no standard agreement whether to use + or − in the definition).

Short map is a distance non increasing map.

Smooth manifold

Sol manifold is a factor of a connected solvable Lie group by a lattice.

Submetry a short map ''f'' between metric spaces is called a submetry if there exists ''R > 0'' such that for any point ''x'' and radius ''r < R'' we have that image of metric ''r''-ball is an ''r''-ball, i.e.
:$f(B_r(x))=B_r(f(x))$

Sub-Riemannian manifold

Systole. The ''k''-systole of ''M'', $syst_k(M)$, is the minimal volume of ''k''-cycle nonhomologous to zero.

T

Tangent bundle

Totally convex. A subset ''K'' of a Riemannian manifold ''M'' is called totally convex if for any two points in ''K'' any geodesic connecting them lies entirely in ''K'', see also ''convex''.

Totally geodesic submanifold is a ''submanifold'' such that all '' geodesics'' in the submanifold are also geodesics of the surrounding manifold.

U

Uniquely geodesic metric space is a metric space where any two points are the endpoints of a unique minimizing geodesic.

W

Word metric on a group is a metric of the Cayley graph constructed using a set of generators.

{{DEFAULTSORT:Glossary Of Riemannian And Metric Geometry

Differential geometry
Geometry
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