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In Riemannian geometry, an **exponential map** is a map from a subset of a tangent space T_{p}*M* of a Riemannian manifold (or pseudo-Riemannian manifold) *M* to *M* itself. The (pseudo) Riemannian metric determines a canonical affine connection, and the exponential map of the (pseudo) Riemannian manifold is given by the exponential map of this connection.

**Riemannian geometry** is the branch of differential geometry that studies Riemannian manifolds, smooth manifolds with a *Riemannian metric*, i.e. with an inner product on the tangent space at each point that varies smoothly from point to point. This gives, in particular, local notions of angle, length of curves, surface area and volume. From those, some other global quantities can be derived by integrating local contributions.

In mathematics, the **tangent space** of a manifold facilitates the generalization of vectors from affine spaces to general manifolds, since in the latter case one cannot simply subtract two points to obtain a vector that gives the displacement of the one point from the other.

In differential geometry, a (**smooth**) **Riemannian manifold** or (**smooth**) **Riemannian space**(*M*, *g*) is a real, smooth manifold *M* equipped with an inner product *g*_{p} on the tangent space *T*_{p}*M* at each point *p* that varies smoothly from point to point in the sense that if *X* and *Y* are differentiable vector fields on *M*, then *p* ↦ *g*_{p}(*X*|_{p}, *Y*|_{p}) is a smooth function. The family *g*_{p} of inner products is called a Riemannian metric. These terms are named after the German mathematician Bernhard Riemann. The study of Riemannian manifolds constitutes the subject called Riemannian geometry.

Let *M* be a differentiable manifold and *p* a point of *M*. An affine connection on *M* allows one to define the notion of a straight line through the point *p*.^{ [1] }

In mathematics, a **differentiable manifold** is a type of manifold that is locally similar enough to a linear space to allow one to do calculus. Any manifold can be described by a collection of charts, also known as an atlas. One may then apply ideas from calculus while working within the individual charts, since each chart lies within a linear space to which the usual rules of calculus apply. If the charts are suitably compatible, then computations done in one chart are valid in any other differentiable chart.

In the branch of mathematics called differential geometry, an **affine connection** is a geometric object on a smooth manifold which *connects* nearby tangent spaces, so it permits tangent vector fields to be differentiated as if they were functions on the manifold with values in a fixed vector space. The notion of an affine connection has its roots in 19th-century geometry and tensor calculus, but was not fully developed until the early 1920s, by Élie Cartan and Hermann Weyl. The terminology is due to Cartan and has its origins in the identification of tangent spaces in Euclidean space **R**^{n} by translation: the idea is that a choice of affine connection makes a manifold look infinitesimally like Euclidean space not just smoothly, but as an affine space.

Let *v* ∈ T_{p}*M* be a tangent vector to the manifold at *p*. Then there is a unique geodesic *γ*_{v} satisfying *γ*_{v}(0) = *p* with initial tangent vector *γ*′_{v}(0) = *v*. The corresponding **exponential map** is defined by exp_{p}(*v*) = *γ*_{v}(1). In general, the exponential map is only *locally defined*, that is, it only takes a small neighborhood of the origin at T_{p}*M*, to a neighborhood of *p* in the manifold. This is because it relies on the theorem of existence and uniqueness for ordinary differential equations which is local in nature. An affine connection is called complete if the exponential map is well-defined at every point of the tangent bundle.

In mathematics, a **tangent vector** is a vector that is tangent to a curve or surface at a given point. Tangent vectors are described in the differential geometry of curves in the context of curves in **R**^{n}. More generally, tangent vectors are elements of a tangent space of a differentiable manifold. Tangent vectors can also be described in terms of germs. Formally, a tangent vector at the point is a linear derivation of the algebra defined by the set of germs at .

In differential geometry, a **geodesic** is a generalization of the notion of a "straight line" to "curved spaces". The term "geodesic" comes from *geodesy*, the science of measuring the size and shape of Earth; in the original sense, a geodesic was the shortest route between two points on the Earth's surface, namely, a segment of a great circle. The term has been generalized to include measurements in much more general mathematical spaces; for example, in graph theory, one might consider a geodesic between two vertices/nodes of a graph.

In mathematics – specifically, in differential equations – the **Picard–Lindelöf theorem**, **Picard's existence theorem**, **Cauchy–Lipschitz theorem**, or **existence and uniqueness theorem** gives a set of conditions under which an initial value problem has a unique solution.

Intuitively speaking, the exponential map takes a given tangent vector to the manifold, runs along the geodesic starting at that point and goes in that direction, for a unit time. Since *v* corresponds to the velocity vector of the geodesic, the actual (Riemannian) distance traveled will be dependent on that. We can also reparametrize geodesics to be unit speed, so equivalently we can define exp_{p}(*v*) = β(|*v*|) where β is the unit-speed geodesic (geodesic parameterized by arc length) going in the direction of *v*. As we vary the tangent vector *v* we will get, when applying exp_{p}, different points on *M* which are within some distance from the base point *p*—this is perhaps one of the most concrete ways of demonstrating that the tangent space to a manifold is a kind of "linearization" of the manifold.

The Hopf–Rinow theorem asserts that it is possible to define the exponential map on the whole tangent space if and only if the manifold is complete as a metric space (which justifies the usual term **geodesically complete** for a manifold having an exponential map with this property). In particular, compact manifolds are geodesically complete. However even if exp_{p} is defined on the whole tangent space, it will in general not be a global diffeomorphism. However, its differential at the origin of the tangent space is the identity map and so, by the inverse function theorem we can find a neighborhood of the origin of T_{p}*M* on which the exponential map is an embedding (i.e., the exponential map is a local diffeomorphism). The radius of the largest ball about the origin in T_{p}*M* that can be mapped diffeomorphically via exp_{p} is called the ** injectivity radius ** of *M* at *p*. The cut locus of the exponential map is, roughly speaking, the set of all points where the exponential map fails to have a unique minimum.

**Hopf–Rinow theorem** is a set of statements about the geodesic completeness of Riemannian manifolds. It is named after Heinz Hopf and his student Willi Rinow, who published it in 1931.

In mathematics, a **metric space** is a set together with a metric on the set. The metric is a function that defines a concept of *distance* between any two members of the set, which are usually called points. The metric satisfies a few simple properties. Informally:

In mathematics, and more specifically in general topology, **compactness** is a property that generalizes the notion of a subset of Euclidean space being closed and bounded. Examples include a closed interval, a rectangle, or a finite set of points. This notion is defined for more general topological spaces than Euclidean space in various ways.

An important property of the exponential map is the following lemma of Gauss (yet another Gauss's lemma): given any tangent vector *v* in the domain of definition of exp_{p}, and another vector *w* based at the tip of *v* (hence *w* is actually in the double-tangent space T_{v}(T_{p}*M*)) and orthogonal to *v*, remains orthogonal to *v* when pushed forward via the exponential map. This means, in particular, that the boundary sphere of a small ball about the origin in T_{p}*M* is orthogonal to the geodesics in *M* determined by those vectors (i.e., the geodesics are *radial*). This motivates the definition of geodesic normal coordinates on a Riemannian manifold.

In Riemannian geometry, **Gauss's lemma** asserts that any sufficiently small sphere centered at a point in a Riemannian manifold is perpendicular to every geodesic through the point. More formally, let *M* be a Riemannian manifold, equipped with its Levi-Civita connection, and *p* a point of *M*. The exponential map is a mapping from the tangent space at *p* to *M*:

In mathematics, particularly differential topology, the **double tangent bundle** or the **second tangent bundle** refers to the tangent bundle (*TTM*,*π*_{TTM},*TM*) of the total space *TM* of the tangent bundle (*TM*,*π*_{TM},*M*) of a smooth manifold *M* . A note on notation: in this article, we denote projection maps by their domains, e.g., *π*_{TTM} : *TTM* → *TM*. Some authors index these maps by their ranges instead, so for them, that map would be written *π*_{TM}.

The exponential map is also useful in relating the abstract definition of curvature to the more concrete realization of it originally conceived by Riemann himself—the sectional curvature is intuitively defined as the Gaussian curvature of some surface (i.e., a slicing of the manifold by a 2-dimensional submanifold) through the point *p* in consideration. Via the exponential map, it now can be precisely defined as the Gaussian curvature of a surface through *p* determined by the image under exp_{p} of a 2-dimensional subspace of T_{p}*M*.

In mathematics, specifically differential geometry, the infinitesimal geometry of Riemannian manifolds with dimension greater than 2 is too complicated to be described by a single number at a given point. Riemann introduced an abstract and rigorous way to define curvature for these manifolds, now known as the Riemann curvature tensor. Similar notions have found applications everywhere in differential geometry.

In Riemannian geometry, the **sectional curvature** is one of the ways to describe the curvature of Riemannian manifolds. The sectional curvature *K*(σ_{p}) depends on a two-dimensional plane σ_{p} in the tangent space at a point *p* of the manifold. It is the Gaussian curvature of the surface which has the plane σ_{p} as a tangent plane at *p*, obtained from geodesics which start at *p* in the directions of σ_{p}. The sectional curvature is a smooth real-valued function on the 2-Grassmannian bundle over the manifold.

In differential geometry, the **Gaussian curvature** or **Gauss curvature***Κ* of a surface at a point is the product of the principal curvatures, *κ*_{1} and *κ*_{2}, at the given point:

In the case of Lie groups with a **bi-invariant metric**—a pseudo-Riemannian metric invariant under both left and right translation—the exponential maps of the pseudo-Riemannian structure are the same as the exponential maps of the Lie group. In general, Lie groups do not have a bi-invariant metric, though all connected semi-simple (or reductive) Lie groups do. The existence of a bi-invariant *Riemannian* metric is stronger than that of a pseudo-Riemannian metric, and implies that the Lie algebra is the Lie algebra of a compact Lie group; conversely, any compact (or abelian) Lie group has such a Riemannian metric.

Take the example that gives the "honest" exponential map. Consider the positive real numbers **R**^{+}, a Lie group under the usual multiplication. Then each tangent space is just **R**. On each copy of **R** at the point *y*, we introduce the modified inner product

(multiplying them as usual real numbers but scaling by *y*^{2}). (This is what makes the metric left-invariant, for left multiplication by a factor will just pull out of the inner product, twice — canceling the square in the denominator).

Consider the point 1 ∈ **R**^{+}, and *x* ∈ **R** an element of the tangent space at 1. The usual straight line emanating from 1, namely *y*(*t*) = 1 + *xt* covers the same path as a geodesic, of course, except we have to reparametrize so as to get a curve with constant speed ("constant speed", remember, is not going to be the ordinary constant speed, because we're using this funny metric). To do this we reparametrize by arc length (the integral of the length of the tangent vector in the norm induced by the modified metric):

and after inverting the function to obtain *t* as a function of *s*, we substitute and get

Now using the unit speed definition, we have

- ,

giving the expected *e*^{x}.

The Riemannian distance defined by this is simply

- ,

a metric which should be familiar to anyone who has drawn graphs on log paper.

- ↑ A source for this section is Kobayashi & Nomizu (1975 , §III.6), which uses the term "linear connection" where we use "affine connection" instead.

In the mathematical field of differential geometry, the **Riemann curvature tensor** or **Riemann–Christoffel tensor** is the most common method used to express the curvature of Riemannian manifolds. It assigns a tensor to each point of a Riemannian manifold, that measures the extent to which the metric tensor is not locally isometric to that of Euclidean space. The curvature tensor can also be defined for any pseudo-Riemannian manifold, or indeed any manifold equipped with an affine connection.

In Riemannian geometry, the **Levi-Civita connection** is a specific connection on the tangent bundle of a manifold. More specifically, it is the torsion-free metric connection, i.e., the torsion-free connection on the tangent bundle preserving a given (pseudo-)Riemannian metric.

In differential geometry, the **Ricci curvature tensor**, named after Gregorio Ricci-Curbastro, represents the amount by which the volume of a narrow conical piece of a small geodesic ball in a curved Riemannian manifold deviates from that of the standard ball in Euclidean space. As such, it provides one way of measuring the degree to which the geometry determined by a given Riemannian metric might differ from that of ordinary Euclidean n-space. The Ricci tensor is defined on any pseudo-Riemannian manifold, as a trace of the Riemann curvature tensor. Like the metric itself, the Ricci tensor is a symmetric bilinear form on the tangent space of the manifold.

In geometry, **parallel transport** is a way of transporting geometrical data along smooth curves in a manifold. If the manifold is equipped with an affine connection, then this connection allows one to transport vectors of the manifold along curves so that they stay *parallel* with respect to the connection.

In differential geometry, a **pseudo-Riemannian manifold**, also called a **semi-Riemannian manifold**, is a differentiable manifold with a metric tensor that is everywhere nondegenerate. This is a generalization of a Riemannian manifold in which the requirement of positive-definiteness is relaxed.

In mathematics, particularly differential geometry, a **Finsler manifold** is a differentiable manifold *M* where a **Minkowski functional***F*(*x*,−) is provided on each tangent space T_{x}*M*, allowing to define the length of any smooth curve *γ* : [*a*,*b*] → *M* as

In mathematics, the **covariant derivative** is a way of specifying a derivative along tangent vectors of a manifold. Alternatively, the covariant derivative is a way of introducing and working with a connection on a manifold by means of a differential operator, to be contrasted with the approach given by a principal connection on the frame bundle – see affine connection. In the special case of a manifold isometrically embedded into a higher-dimensional Euclidean space, the covariant derivative can be viewed as the orthogonal projection of the Euclidean derivative along a tangent vector onto the manifold's tangent space. In this case the Euclidean derivative is broken into two parts, the extrinsic normal component and the intrinsic covariant derivative component.

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

In mathematics, a **metric** or **distance function** is a function that defines a distance between each pair of elements of a set. A set with a metric is called a metric space. A metric induces a topology on a set, but not all topologies can be generated by a metric. A topological space whose topology can be described by a metric is called metrizable.

In differential geometry, representation theory and harmonic analysis, a **symmetric space** is a pseudo-Riemannian manifold whose group of symmetries contains an inversion symmetry about every point. This can be made more precise, in either the language of Riemannian geometry or of Lie theory. The Riemannian definition is more geometric, and plays a deep role in the theory of holonomy. The Lie-theoretic definition is more algebraic.

The **mathematics of general relativity** refers to various mathematical structures and techniques that are used in studying and formulating Albert Einstein's theory of general relativity. The main tools used in this geometrical theory of gravitation are tensor fields defined on a Lorentzian manifold representing spacetime. This article is a general description of the mathematics of general relativity.

In mathematics, more particularly in the fields of dynamical systems and geometric topology, an **Anosov map** on a manifold *M* is a certain type of mapping, from *M* to itself, with rather clearly marked local directions of "expansion" and "contraction". Anosov systems are a special case of Axiom A systems.

In mathematics, the **vector flow** refers to a set of closely related concepts of the flow determined by a vector field. These appear in a number of different contexts, including differential topology, Riemannian geometry and Lie group theory. These related concepts are explored in a spectrum of articles:

In differential geometry, **normal coordinates** at a point *p* in a differentiable manifold equipped with a symmetric affine connection are a local coordinate system in a neighborhood of *p* obtained by applying the exponential map to the tangent space at *p*. In a normal coordinate system, the Christoffel symbols of the connection vanish at the point *p*, thus often simplifying local calculations. In normal coordinates associated to the Levi-Civita connection of a Riemannian manifold, one can additionally arrange that the metric tensor is the Kronecker delta at the point *p*, and that the first partial derivatives of the metric at *p* vanish.

In differential geometry, the notion of a metric tensor can be extended to an arbitrary vector bundle, and to some principal fiber bundles. This metric is often called a **bundle metric**, or **fibre metric**.

In mathematics, the **Riemannian connection on a surface** or Riemannian 2-manifold refers to several intrinsic geometric structures discovered by Tullio Levi-Civita, Élie Cartan and Hermann Weyl in the early part of the twentieth century: parallel transport, covariant derivative and connection form. These concepts were put in their current form with principal bundles only in the 1950s. The classical nineteenth century approach to the differential geometry of surfaces, due in large part to Carl Friedrich Gauss, has been reworked in this modern framework, which provides the natural setting for the classical theory of the moving frame as well as the Riemannian geometry of higher-dimensional Riemannian manifolds. This account is intended as an introduction to the theory of connections.

In the theory of Lie groups, the **exponential map** is a map from the Lie algebra of a Lie group to the group, which allows one to recapture the local group structure from the Lie algebra. The existence of the exponential map is one of the primary reasons that Lie algebras are a useful tool for studying Lie groups.

- do Carmo, Manfredo P. (1992),
*Riemannian Geometry*, Birkhäuser, ISBN 0-8176-3490-8 . See Chapter 3. - Cheeger, Jeff; Ebin, David G. (1975),
*Comparison Theorems in Riemannian Geometry*, Elsevier. See Chapter 1, Sections 2 and 3. - Hazewinkel, Michiel, ed. (2001) [1994], "Exponential mapping",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - Helgason, Sigurdur (2001),
*Differential geometry, Lie groups, and symmetric spaces*, Graduate Studies in Mathematics,**34**, Providence, R.I.: American Mathematical Society, ISBN 978-0-8218-2848-9, MR 1834454 . - Kobayashi, Shoshichi; Nomizu, Katsumi (1996),
*Foundations of Differential Geometry*, Vol. 1 (New ed.), Wiley-Interscience, ISBN 0-471-15733-3 .

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