Differential geometry

**Differential geometry** is a mathematical discipline that uses the techniques of differential calculus, integral calculus, linear algebra and multilinear algebra to study problems in geometry. The theory of plane and space curves and surfaces in the three-dimensional Euclidean space formed the basis for development of differential geometry during the 18th century and the 19th century.

Since the late 19th century, differential geometry has grown into a field concerned more generally with the geometric structures on differentiable manifolds. Differential geometry is closely related to differential topology and the geometric aspects of the theory of differential equations. The differential geometry of surfaces captures many of the key ideas and techniques endemic to this field.

## History of development

Differential geometry arose and developed as a result of and in connection to the mathematical analysis of curves and surfaces. Mathematical analysis of curves and surfaces had been developed to answer some of the nagging and unanswered questions that appeared in calculus, like the reasons for relationships between complex shapes and curves, series and analytic functions. These unanswered questions indicated greater, hidden relationships.

The general idea of natural equations for obtaining curves from local curvature appears to have been first considered by Leonhard Euler in 1736, and many examples with fairly simple behavior were studied in the 1800s.

When curves, surfaces enclosed by curves, and points on curves were found to be quantitatively, and generally, related by mathematical forms, the formal study of the nature of curves and surfaces became a field of study in its own right, with Monge's paper in 1795, and especially, with Gauss's publication of his article, titled 'Disquisitiones Generales Circa Superficies Curvas', in *Commentationes Societatis Regiae Scientiarum Gottingesis Recentiores* in 1827.

Initially applied to the Euclidean space, further explorations led to non-Euclidean space, and metric and topological spaces.

## Branches

### Riemannian geometry

Main articles: Riemannian geometry
Riemannian geometry studies Riemannian manifolds, smooth manifolds with a *Riemannian metric*. This is a concept of distance expressed by means of a smooth positive definite symmetric bilinear form defined on the tangent space at each point. Riemannian geometry generalizes Euclidean geometry to spaces that are not necessarily flat, although they still resemble the Euclidean space at each point infinitesimally, i.e. in the first order of approximation. Various concepts based on length, such as the arc length of curves, area of plane regions, and volume of solids all possess natural analogues in Riemannian geometry. The notion of a directional derivative of a function from multivariable calculus is extended in Riemannian geometry to the notion of a covariant derivative of a tensor. Many concepts and techniques of analysis and differential equations have been generalized to the setting of Riemannian manifolds.

A distance-preserving diffeomorphism between Riemannian manifolds is called an isometry. This notion can also be defined *locally*, i.e. for small neighborhoods of points. Any two regular curves are locally isometric. However, the Theorema Egregium of Carl Friedrich Gauss showed that for surfaces, the existence of a local isometry imposes strong compatibility conditions on their metrics: the Gaussian curvatures at the corresponding points must be the same. In higher dimensions, the Riemann curvature tensor is an important pointwise invariant associated with a Riemannian manifold that measures how close it is to being flat. An important class of Riemannian manifolds is the Riemannian symmetric spaces, whose curvature is not necessarily constant. These are the closest analogues to the "ordinary" plane and space considered in Euclidean and non-Euclidean geometry.

### Pseudo-Riemannian geometry

Pseudo-Riemannian geometry generalizes Riemannian geometry to the case in which the metric tensor need not be positive-definite.
A special case of this is a Lorentzian manifold, which is the mathematical basis of Einstein's general relativity theory of gravity.

### Finsler geometry

Main articles: Finsler manifold
Finsler geometry has *Finsler manifolds* as the main object of study. This is a differential manifold with a *Finsler metric*, that is, a Banach norm defined on each tangent space. Riemannian manifolds are special cases of the more general Finsler manifolds. A Finsler structure on a manifold *M* is a function *F* : T*M* → [0, ∞) such that:

1. *F*(*x*, *my*) = *m* *F*(*x*, *y*) for all (*x*, *y*) in T*M* and all *m*≥0,
2. *F* is infinitely differentiable in T*M* ∖ {0},
3. The vertical Hessian of *F*2 is positive definite.

### Symplectic geometry

Main articles: Symplectic geometry
Symplectic geometry is the study of symplectic manifolds. An **almost symplectic manifold** is a differentiable manifold equipped with a smoothly varying non-degenerate skew-symmetric bilinear form on each tangent space, i.e., a nondegenerate 2-form *ω*, called the *symplectic form*. A symplectic manifold is an almost symplectic manifold for which the symplectic form *ω* is closed: d*ω* = 0.

A diffeomorphism between two symplectic manifolds which preserves the symplectic form is called a symplectomorphism. Non-degenerate skew-symmetric bilinear forms can only exist on even-dimensional vector spaces, so symplectic manifolds necessarily have even dimension. In dimension 2, a symplectic manifold is just a surface endowed with an area form and a symplectomorphism is an area-preserving diffeomorphism. The phase space of a mechanical system is a symplectic manifold and they made an implicit appearance already in the work of Joseph Louis Lagrange on analytical mechanics and later in Carl Gustav Jacobi's and William Rowan Hamilton's formulations of classical mechanics.

By contrast with Riemannian geometry, where the curvature provides a local invariant of Riemannian manifolds, Darboux's theorem states that all symplectic manifolds are locally isomorphic. The only invariants of a symplectic manifold are global in nature and topological aspects play a prominent role in symplectic geometry. The first result in symplectic topology is probably the Poincaré–Birkhoff theorem, conjectured by Henri Poincaré and then proved by G.D. Birkhoff in 1912. It claims that if an area preserving map of an annulus twists each boundary component in opposite directions, then the map has at least two fixed points.

### Contact geometry

Main articles: Contact geometry
Contact geometry deals with certain manifolds of odd dimension. It is close to symplectic geometry and like the latter, it originated in questions of classical mechanics. A *contact structure* on a (2*n* + 1)-dimensional manifold *M* is given by a smooth hyperplane field *H* in the tangent bundle that is as far as possible from being associated with the level sets of a differentiable function on *M* (the technical term is "completely nonintegrable tangent hyperplane distribution"). Near each point *p*, a hyperplane distribution is determined by a nowhere vanishing 1-form

α

{\displaystyle \alpha}

, which is unique up to multiplication by a nowhere vanishing function:

H
p

=
ker
⁡

α
p

⊂

T

p

M
.

{\displaystyle H\_p = \ker\alpha\_p\subset T\_{p}M.}

A local 1-form on *M* is a *contact form* if the restriction of its exterior derivative to *H* is a non-degenerate two-form and thus induces a symplectic structure on *H**p* at each point. If the distribution *H* can be defined by a global one-form

α

{\displaystyle \alpha}

 then this form is contact if and only if the top-dimensional form

α
∧
(
d
α

)
n

{\displaystyle \alpha\wedge (d\alpha)^n}

is a volume form on *M*, i.e. does not vanish anywhere. A contact analogue of the Darboux theorem holds: all contact structures on an odd-dimensional manifold are locally isomorphic and can be brought to a certain local normal form by a suitable choice of the coordinate system.

### Complex and Kähler geometry

See also: Complex geometry
*Complex differential geometry* is the study of complex manifolds.
An almost complex manifold is a *real* manifold

M

{\displaystyle M}

, endowed with a tensor of type (1, 1), i.e. a vector bundle endomorphism (called an *almost complex structure*)

J
:
T
M
→
T
M

{\displaystyle J:TM\rightarrow TM }

, such that

J
2

=
−

{\displaystyle J^2=-1. \,}

It follows from this definition that an almost complex manifold is even-dimensional.

An almost complex manifold is called *complex* if

N
J

=
0

{\displaystyle N\_J=0}

, where

N
J

{\displaystyle N\_J}

 is a tensor of type (2, 1) related to

J

{\displaystyle J}

, called the Nijenhuis tensor (or sometimes the *torsion*).
An almost complex manifold is complex if and only if it admits a holomorphic coordinate atlas.
An *almost Hermitian structure* is given by an almost complex structure *J*, along with a Riemannian metric *g*, satisfying the compatibility condition

g
(
J
X
,
J
Y
# )
g
(
X
,
Y
)

{\displaystyle g(JX,JY)=g(X,Y) \,}

.
An almost Hermitian structure defines naturally a differential two-form

ω

J
,
g

(
X
,
Y
)
:=
g
(
J
X
,
Y
)

{\displaystyle \omega\_{J,g}(X,Y):=g(JX,Y) \,}

.
The following two conditions are equivalent:

1. N
J

=
0

 and

d
# ω
0

{\displaystyle N\_J=0\mbox{ and }d\omega=0 \,}
2. ∇
# J
0

{\displaystyle \nabla J=0 \,}

where

∇

{\displaystyle \nabla }

 is the Levi-Civita connection of

g

{\displaystyle g}

. In this case,

(
J
,
g
)

{\displaystyle (J, g)}

 is called a *Kähler structure*, and a *Kähler manifold* is a manifold endowed with a Kähler structure. In particular, a Kähler manifold is both a complex and a symplectic manifold. A large class of Kähler manifolds (the class of Hodge manifolds) is given by all the smooth complex projective varieties.

### CR geometry

CR geometry is the study of the intrinsic geometry of boundaries of domains in complex manifolds.

### Differential topology

Differential topology is the study of global geometric invariants without a metric or symplectic form.

Differential topology starts from the natural operations such as Lie derivative of natural vector bundles and de Rham differential of forms. Beside Lie algebroids, also Courant algebroids start playing a more important role.

### Lie groups

A Lie group is a group in the category of smooth manifolds. Beside the algebraic properties this enjoys also differential geometric properties. The most obvious construction is that of a Lie algebra which is the tangent space at the unit endowed with the Lie bracket between left-invariant vector fields. Beside the structure theory there is also the wide field of representation theory.

## Bundles and connections

The apparatus of vector bundles, principal bundles, and connections on bundles plays an extraordinarily important role in modern differential geometry. A smooth manifold always carries a natural vector bundle, the tangent bundle. Loosely speaking, this structure by itself is sufficient only for developing analysis on the manifold, while doing geometry requires, in addition, some way to relate the tangent spaces at different points, i.e. a notion of parallel transport. An important example is provided by affine connections. For a surface in **R**3, tangent planes at different points can be identified using a natural path-wise parallelism induced by the ambient Euclidean space, which has a well-known standard definition of metric and parallelism. In Riemannian geometry, the Levi-Civita connection serves a similar purpose. (The Levi-Civita connection defines path-wise parallelism in terms of a given arbitrary Riemannian metric on a manifold.) More generally, differential geometers consider spaces with a vector bundle and an arbitrary affine connection which is not defined in terms of a metric. In physics, the manifold may be the space-time continuum and the bundles and connections are related to various physical fields.

## Intrinsic versus extrinsic

From the beginning and through the middle of the 18th century, differential geometry was studied from the *extrinsic* point of view: curves and surfaces were considered as lying in a Euclidean space of higher dimension (for example a surface in an ambient space of three dimensions). The simplest results are those in the differential geometry of curves and differential geometry of surfaces. Starting with the work of Riemann, the *intrinsic* point of view was developed, in which one cannot speak of moving "outside" the geometric object because it is considered to be given in a free-standing way. The fundamental result here is Gauss's theorema egregium, to the effect that Gaussian curvature is an intrinsic invariant.

The intrinsic point of view is more flexible. For example, it is useful in relativity where space-time cannot naturally be taken as extrinsic (what would be "outside" of the universe?). However, there is a price to pay in technical complexity: the intrinsic definitions of curvature and connections become much less visually intuitive.

These two points of view can be reconciled, i.e. the extrinsic geometry can be considered as a structure additional to the intrinsic one. (See the Nash embedding theorem.) In the formalism of geometric calculus both extrinsic and intrinsic geometry of a manifold can be characterized by a single bivector-valued one-form called the shape operator.

## Applications

Below are some examples of how differential geometry is applied to other fields of science and mathematics.

* In physics, differential geometry has many applications, including:
	+ Differential geometry is the language in which Einstein's general theory of relativity is expressed. According to the theory, the universe is a smooth manifold equipped with a pseudo-Riemannian metric, which describes the curvature of space-time. Understanding this curvature is essential for the positioning of satellites into orbit around the earth. Differential geometry is also indispensable in the study of gravitational lensing and black holes.
	+ Differential forms are used in the study of electromagnetism.
	+ Differential geometry has applications to both Lagrangian mechanics and Hamiltonian mechanics. Symplectic manifolds in particular can be used to study Hamiltonian systems.
	+ Riemannian geometry and contact geometry have been used to construct the formalism of geometrothermodynamics which has found applications in classical equilibrium thermodynamics.
* In chemistry and biophysics when modelling cell membrane structure under varying pressure.
* In economics, differential geometry has applications to the field of econometrics.
* Geometric modeling (including computer graphics) and computer-aided geometric design draw on ideas from differential geometry.
* In engineering, differential geometry can be applied to solve problems in digital signal processing.
* In control theory, differential geometry can be used to analyze nonlinear controllers, particularly geometric control
* In probability, statistics, and information theory, one can interpret various structures as Riemannian manifolds, which yields the field of information geometry, particularly via the Fisher information metric.
* In structural geology, differential geometry is used to analyze and describe geologic structures.
* In computer vision, differential geometry is used to analyze shapes.
* In image processing, differential geometry is used to process and analyse data on non-flat surfaces.
* Grigori Perelman's proof of the Poincaré conjecture using the techniques of Ricci flows demonstrated the power of the differential-geometric approach to questions in topology and it highlighted the important role played by its analytic methods.
* In wireless communications,  Grassmannian manifolds are used for beamforming techniques in multiple antenna systems.