Manifold 2073470 225885632 2008-07-15T21:25:07Z Pmanderson 64669 /* Other curves */ idiom [[Image:Triangles (spherical geometry).jpg|thumb|300px|On a sphere, the sum of the [[angle|angles]] of a [[triangle]] is not equal to 180° (see [[spherical trigonometry]]). A sphere is not a Euclidean space, but locally the laws of Euclidean geometry are good approximations. In a small triangle on the face of the earth, the sum of the angles is very nearly 180°. A sphere can be represented by a collection of two dimensional maps, therefore a sphere is a manifold.]] {{otheruses3|Manifold (disambiguation)}} A '''manifold''' is an abstract [[topological space|mathematical space]] in which every point has a [[neighborhood (mathematics)|neighborhood]] which resembles [[Euclidean space]], but in which the global structure may be more complicated. In discussing manifolds, the idea of [[dimension]] is important. For example, [[line (mathematics)|lines]] are one-dimensional, and [[plane (mathematics)|planes]] two-dimensional. In a one-dimensional manifold (or one-manifold), every point has a neighborhood that looks like a segment of a line. Examples of one-manifolds include a line, a [[circle]], and two separate circles. In a two-manifold, every point has a neighborhood that looks like a [[disk (mathematics)|disk]]. Examples include a plane, the surface of a [[sphere]], and the surface of a [[torus]]. Manifolds are important objects in mathematics and [[physics]] because they allow more complicated structures to be expressed and understood in terms of the relatively well-understood properties of simpler spaces. Additional structures are often defined on manifolds. Examples of manifolds with additional structure include [[differentiable manifold]]s on which one can do [[calculus]], [[Riemannian manifold]]s on which distances and angles can be defined, [[symplectic manifold]]s which serve as the [[phase space]] in [[classical mechanics]], and the four-dimensional [[pseudo-Riemannian manifold]]s which model [[space-time]] in [[general relativity]]. A precise mathematical definition of a manifold is given below. To fully understand the mathematics behind manifolds, it is necessary to know elementary concepts regarding [[set (mathematics)|set]]s and [[function (mathematics)|function]]s, and helpful to have a working knowledge of [[calculus]] and [[topology]]. == Motivational examples == === Circle === [[Image:Circle with overlapping manifold charts.svg|right|thumb|Figure 1: The four charts each map part of the circle to an open interval, and together cover the whole circle.]] The [[circle]] is the simplest example of a topological manifold after a line. Topology ignores bending, so a small piece of a circle is exactly the same as a small piece of a line. Consider, for instance, the top half of the [[unit circle]], ''x''<sup>2</sup> + ''y''<sup>2</sup> = 1, where the [[Cartesian coordinate system|''y''-coordinate]] is positive (indicated by the yellow arc in ''Figure 1''). Any point of this semicircle can be uniquely described by its ''x''-coordinate. So, [[Projection (mathematics)|projection]] onto the first coordinate is a [[Continuous function (topology)|continuous]], and [[inverse function|invertible]], [[mapping (mathematics)|mapping]] from the upper semicircle to the [[open interval]] (&minus;1,1): :<math> \chi_{\mathrm{top}}(x,y) = x . \,\! </math> Such functions along with the open regions they map are called ''chart''s. Similarly, there are charts for the bottom (red), left (blue), and right (green) parts of the circle. Together, these parts cover the whole circle and the four charts form an [[Atlas (topology)|atlas]] for the circle. The top and right charts overlap: their intersection lies in the quarter of the circle where both the ''x''- and the ''y''-coordinates are positive. The two charts χ<sub>top</sub> and χ<sub>right</sub> each map this part into the interval (0,1). Thus a function ''T'' from (0,1) to itself can be constructed, which first uses the [[inverse function|inverse]] of the top chart to reach the circle and then follows the right chart back to the interval. Let ''a'' be any number in (0,1), then: :<math>\begin{align} T(a) &= \chi_{\mathrm{right}}\left(\chi_{\mathrm{top}}^{-1}(a)\right) \\ &= \chi_{\mathrm{right}}\left(a, \sqrt{1-a^2}\right) \\ &= \sqrt{1-a^2} . \end{align}</math> Such a function is called a ''transition map''. [[Image:Circle manifold chart from slope.svg|right|thumb|Figure 2: A circle manifold chart based on slope, covering all but one point of the circle.]] The top, bottom, left, and right charts show that the circle is a manifold, but they do not form the only possible atlas. Charts need not be geometric projections, and the number of charts is a matter of some choice. Consider the charts : <math>\chi_{\mathrm{minus}}(x,y) = s = \frac{y}{1+x}</math> and : <math>\chi_{\mathrm{plus}}(x,y) = t = \frac{y}{1-x}.</math> Here ''s'' is the slope of the line through the point at coordinates (''x'',''y'') and the fixed pivot point (−1,0); ''t'' is the mirror image, with pivot point (+1,0). The inverse mapping from ''s'' to (''x'',''y'') is given by : <math>\begin{align} x &= \frac{1-s^2}{1+s^2} \\ y &= \frac{2s}{1+s^2} . \end{align}</math> It can easily be confirmed that ''x''<sup>2</sup>+''y''<sup>2</sup>&nbsp;= 1 for all values of the slope ''s''. These two charts provide a second atlas for the circle, with : <math>t = \frac{1}{s} . \,\!</math> Each chart omits a single point, either (−1,0) for ''s'' or (+1,0) for ''t'', so neither chart alone is sufficient to cover the whole circle. Topology can prove that it is not possible to cover the full circle with a single chart. For example, although it is possible to construct a circle from a single line interval by overlapping and "glueing" the ends, this does not produce a chart; a portion of the circle will be mapped to both ends at once, losing invertibility. === Other curves === [[Image:Conics and cubic.svg|right|thumb|Four manifolds from algebraic curves: <span style="color:#bc1e47">■</span>&nbsp;circles, <span style="color:#fec200">■</span>&nbsp;parabola, <span style="color:#0081cd">■</span>&nbsp;hyperbola, <span style="color:#009246">■</span>&nbsp;cubic.]] Manifolds need not be [[connected space|connected]] (all in "one piece"); thus a pair of separate circles is also a manifold. They need not be [[closed manifold|closed]]; thus a line segment without its end points is a manifold. And they need not be finite; thus a [[parabola]] is a manifold. Putting these freedoms together, two other examples of manifolds are a [[hyperbola]] (two open, infinite pieces) and the [[locus (mathematics)|locus]] of points on the [[cubic curve]] ''y''<sup>2</sup>&nbsp;=&nbsp;''x''<sup>3</sup>−''x'' (a closed loop piece and an open, infinite piece). However, we exclude examples like two touching circles that share a point to form a figure-8; at the shared point we cannot create a satisfactory chart. Even with the bending allowed by topology, the vicinity of the shared point looks like a "+", not a line. === Enriched circle === Viewed using [[calculus]], the circle transition function ''T'' is simply a function between open intervals, which gives a meaning to the statement that ''T'' is [[derivative|differentiable]]. The transition map ''T'', and all the others, are differentiable on (0, 1); therefore, with this atlas the circle is a ''[[differentiable manifold]]''. It is also ''smooth'' and ''analytic'' because the transition functions have these properties as well. Other circle properties allow it to meet the requirements of more specialized types of manifold. For example, the circle has a notion of distance between two points, the arc-length between the points; hence it is a ''[[Riemannian manifold]]''. == History == {{seedetails|History of manifolds and varieties}} The study of manifolds combines many important areas of mathematics: it generalizes concepts such as [[curve]]s and [[surfaces]] as well as ideas from [[linear algebra]] and [[topology]]. ===Prehistory=== Before the modern concept of a manifold there were several important results. [[Non-Euclidean geometry]] considers spaces where [[Euclid]]'s [[parallel postulate]] fails. [[Giovanni Gerolamo Saccheri|Saccheri]] first studied them in [[1733]]. [[Nikolai Ivanovich Lobachevsky|Lobachevsky]], [[János Bolyai|Bolyai]], and [[Bernhard Riemann|Riemann]] developed them 100 years later. Their research uncovered two types of spaces whose geometric structures differ from that of classical [[Euclidean space]]; these gave rise to [[hyperbolic geometry]] and [[elliptic geometry]]. In the modern theory of manifolds, these notions correspond to [[Riemannian manifold]]s with constant negative and positive [[curvature]], respectively. [[Carl Friedrich Gauss]] may have been the first to consider abstract spaces as mathematical objects in their own right. His [[theorema egregium]] gives a method for computing the [[curvature]] of a [[surface]] without considering the [[ambient space]] in which the surface lies. Such a surface would, in modern terminology, be called a manifold; and in modern terms, the theorem proved that the curvature of the surface is an intrinsic property. Manifold theory has come to focus exclusively on these intrinsic properties (or invariants), while largely ignoring the extrinsic properties of the ambient space. Another, more [[topology|topological]] example of an intrinsic [[topological property|property]] of a manifold is its [[Euler characteristic]]. [[Leonhard Euler]] showed that for a convex [[polytope]] in the three-dimensional Euclidean space with ''V'' vertices (or corners), ''E'' edges, and ''F'' faces, : ''V''-''E''+''F''= 2. The same formula will hold if we project the vertices and edges of the polytope onto a [[sphere]], creating a 'map' with ''V'' vertices, ''E'' edges, and ''F'' faces, and in fact, will remain true for any spherical map, even if it does not arise from any convex polytope.<ref>The notion of a map can formalized as a [[cell decomposition]].</ref> Thus 2 is a topological invariant of the sphere, called its '''Euler characteristic'''. On the other hand, a [[torus]] can be sliced open by its 'parallel' and 'meridian' circles, creating a map with ''V''=1 vertex, ''E''=2 edges, and ''F''=1 face. Thus the Euler characteristic of the torus is 1-2+1=0. The Euler characteristic of other surfaces is a useful [[topological invariant]], which can be extended to higher dimensions using [[Betti number]]s. In the mid nineteenth century, the [[Gauss–Bonnet theorem]] linked the Euler characteristic to the Gaussian curvature. ===Synthesis=== Investigations of [[Niels Henrik Abel]] and [[Carl Gustav Jacobi]] on inversion of [[elliptic integral]]s in the first half of 19th century led them to consider special types of [[complex manifold]]s, now known as [[Abelian variety|Jacobians]]. [[Bernhard Riemann]] further contributed to their theory, clarifying the geometric meaning of the process of [[analytic continuation]] of functions of complex variables, although these ideas were way ahead of their time. Another important source of manifolds in 19th century mathematics was [[analytical mechanics]], as developed by [[Simeon Poisson]], Jacobi, and [[William Rowan Hamilton]]. The possible states of a mechanical system are thought to be points of an abstract space, [[phase space]] in [[Lagrangian mechanics|Lagrangian]] and [[Hamiltonian mechanics|Hamiltonian]] formalisms of classical mechanics. This space is, in fact, a high-dimensional manifold, whose [[dimension]] corresponds to the degrees of freedom of the system and where the points are specified by their [[generalized coordinate]]s. For an unconstrained movement of free particles the manifold is equivalent to the Euclidean space, but various [[conservation laws]] constrain it to more complicated formations, e.g. [[Liouville tori]]. The theory of a rotating solid body, developed in the 18th century by [[Leonhard Euler]] and [[Joseph Lagrange]], gives another example where the manifold is nontrivial. Geometrical and topological aspects of classical mechanics were emphasized by [[Henri Poincaré]], one of the founders of [[topology]]. Riemann was the first one to do extensive work generalizing the idea of a surface to higher dimensions. The name ''manifold'' comes from Riemann's original [[German language|German]] term, ''Mannigfaltigkeit'', which [[William Kingdon Clifford]] translated as "manifoldness". In his Göttingen inaugural lecture, Riemann described the set of all possible values of a variable with certain constraints as a ''Mannigfaltigkeit'', because the variable can have ''many'' values. He distinguishes between ''stetige Mannigfaltigkeit'' and ''diskrete'' ''Mannigfaltigkeit'' (''continuous manifoldness'' and ''discontinuous manifoldness''), depending on whether the value changes continuously or not. As continuous examples, Riemann refers to not only colors and the locations of objects in space, but also the possible shapes of a spatial figure. Using [[Mathematical induction|induction]], Riemann constructs an ''n-fach ausgedehnte Mannigfaltigkeit'' (''n times extended manifoldness'' or ''n-dimensional manifoldness'') as a continuous stack of (n−1) dimensional manifoldnesses. Riemann's intuitive notion of a ''Mannigfaltigkeit'' evolved into what is today formalized as a manifold. [[Riemannian manifold]]s and [[Riemann surface]]s are named after [[Bernhard Riemann]]. [[Hermann Weyl]] gave an intrinsic definition for differentiable manifolds in his lecture course on Riemann surfaces in 1911–1912, opening the road to the general concept of a [[topological space]] that followed shortly. During the 1930s [[Hassler Whitney]] and others clarified the [[foundations of mathematics|foundational]] aspects of the subject, and thus intuitions dating back to the latter half of the 19th century became precise, and developed through [[differential geometry]] and [[Lie group]] theory. === Topology of manifolds: highlights === Two-dimensional manifolds, also known as ''surfaces'', were considered by Riemann under the guise of [[Riemann surface]]s, and rigorously classified in the beginning of the 20th century by [[Poul Heegaard]] and [[Max Dehn]]. [[Henri Poincaré]] pioneered the study of three-dimensional manifolds and raised a fundamental question about them, today known as the [[Poincaré conjecture]]. After nearly a century of effort by many mathematicians, starting with Poincaré himself, a consensus among experts (as of 2006) is that [[Grigori Perelman]] has proved the Poincaré conjecture (see the [[Solution of the Poincaré conjecture]]). [[Bill Thurston]]'s [[geometrization conjecture|geometrization program]], formulated in the 1970s, provided a far-reaching extension of the Poincaré conjecture to the general three-dimensional manifolds. Four-dimensional manifolds were brought to the forefront of mathematical research in the 1980s by [[Michael Freedman]] and in a different setting, by [[Simon Donaldson]], who was motivated by the then recent progress in theoretical physics ([[gauge theory|Yang-Mills theory]]), where they serve as a substitute for ordinary 'flat' [[space-time]]. Important work on higher-dimensional manifolds, including analogues of the Poincaré conjecture, had been done earlier by [[René Thom]], [[John Milnor]], [[Stephen Smale]] and [[Sergei Petrovich Novikov|Sergei Novikov]]. One of the most pervasive and flexible techniques underlying much work on the [[differential topology|topology of manifolds]] is [[Morse theory]]. == Mathematical definition == {{details|Categories of manifolds}} Informally, a manifold is a [[topological space|space]] that is "modeled on" [[Euclidean space]]. There are many different kinds of manifolds and generalizations. In [[geometry and topology]], all manifolds are [[topological manifold]]s, possibly with additional structure, most often a [[differentiable manifold|differentiable structure]]. In terms of constructing manifolds via patching, a manifold has an additional structure if the transition maps between different patches satisfy axioms beyond just continuity. For instance, [[differentiable manifold]]s have homeomorphisms on overlapping neighborhoods [[diffeomorphic]] with each other, so that the manifold has a well-defined set of functions which are differentiable in each neighborhood, and so differentiable on the manifold as a whole. Formally, a '''topological manifold'''<ref>In the narrow sense of requiring point-set axioms and finite dimension.</ref> is a [[second countable]] [[Hausdorff space|Hausdorff]] [[topological space|space]] that is [[locally homeomorphic]] to Euclidean space. ''Second countable'' and ''Hausdorff'' are [[point-set topology|point-set]] conditions; ''second countable'' excludes spaces which are in some sense 'too large' such as the [[Long line (topology)|long line]], while ''Hausdorff'' excludes spaces such as "the line with two origins" (these generalized manifolds are discussed in [[non-Hausdorff manifold]]s). ''Locally homeomorphic'' to Euclidean space means<ref>Formally, locally homeomorphic means that each point ''m'' in the manifold ''M'' has a neighborhood homeomorphic to a ''neighborhood'' in Euclidean space, not to the unit ball specifically. However, given such a homeomorphism, the pre-image of an <math>\epsilon</math>-ball gives a homeomorphism between the unit ball and a smaller neighborhood of ''m'', so this is no loss of generality. For topological or differentiable manifolds, one can also ask that every point have a neighborhood homeomorphic to all of Euclidean space (as this is diffeomorphic to the unit ball), but this cannot be done for [[complex manifold]]s, as the complex unit ball is not [[holomorphic]] to complex space.</ref> that every point has a neighborhood [[homeomorphic]] to an open Euclidean ''n''-ball, :<math>\mathbf{B}^n = \{ (x_1, x_2, \dots, x_n)\in\mathbb{R}^n \mid x_1^2 + x_2^2 + \cdots + x_n^2 < 1 \}.</math> Generally manifolds are taken to have a fixed dimension (the space must be locally homeomorphic to a fixed ''n''-ball), and such a space is called an '''''n''-manifold'''; however, some authors admit manifolds where different points can have different [[dimension]]s. Since dimension is a local invariant, each [[connected space|connected component]] has a fixed dimension. [[Scheme (mathematics)|Scheme-theoretically]], a manifold is a [[locally ringed space]], whose structure sheaf is locally isomorphic to the sheaf of continuous (or differentiable, or complex-analytic, etc.) functions on Euclidean space. This definition is mostly used when discussing [[analytic manifold]]s in [[algebraic geometry]]. === Broad definition === The broadest common definition of manifold is a topological space locally homeomorphic to a [[topological vector space]] over the reals. This omits the point-set axioms (allowing higher cardinalities and [[non-Hausdorff manifold]]s) and finite dimension (allowing various manifolds from [[functional analysis]]). Usually one relaxes one or the other condition: manifolds without the point-set axioms are studied in [[general topology]], while infinite-dimensional manifolds are studied in [[functional analysis]]. ==Charts, atlases, and transition maps== The spherical Earth is navigated using flat maps or charts, collected in an atlas. Similarly, a differentiable manifold can be described using [[map (mathematics)|mathematical maps]], called ''coordinate charts'', collected in a mathematical ''atlas''. It is not generally possible to describe a manifold with just one chart, because the global structure of the manifold is different from the simple structure of the charts. For example, no single flat map can properly represent the entire Earth. When a manifold is constructed from multiple overlapping charts, the regions where they overlap carry information essential to understanding the global structure. ===Charts=== A '''coordinate map''', a '''coordinate chart''', or simply a '''chart''', of a manifold is an invertible [[map (mathematics)|map]] between a subset of the manifold and a simple space such that both the map and its inverse preserve the desired structure. For a topological manifold, the simple space is some [[Euclidean space]] '''R'''<sup>''n''</sup> and interest focuses on the topological structure. This structure is preserved by [[homeomorphisms]], invertible maps that are continuous in both directions. In the case of a [[differentiable manifold]], a set of '''charts''' called an '''atlas''' allows us to do calculus on manifolds. [[Polar coordinates]], for example, form a chart for the plane '''R'''<sup>2</sup> minus the positive ''x''-axis and the origin. Another example of a chart is the map χ<sub>top</sub> mentioned in the section above, a chart for the circle. ===Atlases=== The description of most manifolds requires more than one chart (a single chart is adequate for only the simplest manifolds). A specific collection of charts which covers a manifold is called an '''atlas'''. An atlas is not unique as all manifolds can be covered multiple ways using different combinations of charts. The atlas containing all possible charts consistent with a given atlas is called the '''maximal atlas'''. Unlike an ordinary atlas, the maximal atlas of a given manifold is unique. Though it is useful for definitions, it is a very abstract object and not used directly (e.g. in calculations). ===Transition maps=== Charts in an atlas may overlap and a single point of a manifold may be represented in several charts. If two charts overlap, parts of them represent the same region of the manifold, just as a map of Europe and a map of Asia may both contain Moscow. Given two overlapping charts, a '''transition function''' can be defined which goes from an open ball in '''R'''<sup>''n''</sup> to the manifold and then back to another (or perhaps the same) open ball in '''R'''<sup>''n''</sup>. The resultant map, like the map ''T'' in the circle example above, is called a '''change of coordinates''', a '''coordinate transformation''', a '''transition function''', or a '''transition map'''. ===Additional structure=== An atlas can also be used to define additional structure on the manifold. The structure is first defined on each chart separately. If all the transition maps are compatible with this structure, the structure transfers to the manifold. This is the standard way differentiable manifolds are defined. If the transition functions of an atlas for a topological manifold preserve the natural differential structure of '''R'''<sup>''n''</sup> (that is, if they are [[diffeomorphism]]s), the differential structure transfers to the manifold and turns it into a differentiable manifold. [[Complex manifold]]s are introduced in an analogous way by requiring that the transition functions of an atlas are [[holomorphic]] functions. For [[symplectic manifold]]s, the transition functions must be [[symplectomorphism]]s. The structure on the manifold depends on the atlas, but sometimes different atlases can be said to give rise to the same structure. Such atlases are called '''compatible'''. These notions are made precise in general through the use of [[pseudogroup]]s. == Construction == A single manifold can be constructed in different ways, each stressing a different aspect of the manifold, thereby leading to a slightly different viewpoint. === Charts === [[Image:Sphere with chart.svg|right|thumb|The chart maps the part of the sphere with positive ''z'' coordinate to a disc.]] Perhaps the simplest way to construct a manifold is the one used in the example above of the circle. First, a subset of '''R'''<sup>2</sup> is identified, and then an atlas covering this subset is constructed. The concept of ''manifold'' grew historically from constructions like this. Here is another example, applying this method to the construction of a sphere: ==== Sphere with charts ==== A [[sphere]] can be treated in almost the same way as the circle. In mathematics a sphere is just the surface (not the solid interior), which can be defined as a subset of '''R'''<sup>3</sup>: : <math> S = \{ (x,y,z) \in \mathbf{R}^3 | x^2 + y^2 + z^2 = 1 \}. </math> The sphere is two-dimensional, so each chart will map part of the sphere to an open subset of '''R'''<sup>2</sup>. Consider the northern hemisphere, which is the part with positive ''z'' coordinate (coloured red in the picture on the right). The function χ defined by : <big><math> \chi(x,y,z) = (x,y), </math></big> maps the northern hemisphere to the open [[unit disc]] by projecting it on the (''x'', ''y'') plane. A similar chart exists for the southern hemisphere. Together with two charts projecting on the (''x'', ''z'') plane and two charts projecting on the (''y'', ''z'') plane, an atlas of six charts is obtained which covers the entire sphere. This can be easily generalized to higher-dimensional spheres. === Patchwork === A manifold can be constructed by gluing together pieces in a consistent manner, making them into overlapping charts. This construction is possible for any manifold and hence it is often used as a characterisation, especially for differentiable and Riemannian manifolds. It focuses on an atlas, as the patches naturally provide charts, and since there is no exterior space involved it leads to an intrinsic view of the manifold. The manifold is constructed by specifying an atlas, which is itself defined by transition maps. A point of the manifold is therefore an [[equivalence class]] of points which are mapped to each other by transition maps. Charts map equivalence classes to points of a single patch. There are usually strong demands on the consistency of the transition maps. For topological manifolds they are required to be [[homeomorphism]]s; if they are also [[diffeomorphism]]s, the resulting manifold is a differentiable manifold. This can be illustrated with the transition map ''t'' = <sup>1</sup>⁄<sub>''s''</sub> from the second half of the circle example. Start with two copies of the line. Use the coordinate ''s'' for the first copy, and ''t'' for the second copy. Now, glue both copies together by identifying the point ''t'' on the second copy with the point <sup>1</sup>⁄<sub>''s''</sub> on the first copy (the point ''t'' = 0 is not identified with any point on the first copy). This gives a circle. ==== Intrinsic and extrinsic view ==== The first construction and this construction are very similar, but they represent rather different points of view. In the first construction, the manifold is seen as [[embedding|embedded]] in some Euclidean space. This is the ''extrinsic view''. When a manifold is viewed in this way, it is easy to use intuition from Euclidean spaces to define additional structure. For example, in a Euclidean space it is always clear whether a vector at some point is [[tangential]] or [[normal vector|normal]] to some surface through that point. The patchwork construction does not use any embedding, but simply views the manifold as a topological space by itself. This abstract point of view is called the ''intrinsic view''. It can make it harder to imagine what a tangent vector might be. ==== ''n''-Sphere as a patchwork ==== The [[Hypersphere|''n''-sphere]] '''S'''<sup>''n''</sup> is a generalisation of the idea of a circle (1-sphere) and sphere (2-sphere) to higher dimensions. An ''n''-sphere '''S'''<sup>''n''</sup> can be constructed by gluing together two copies of '''R'''<sup>''n''</sup>. The transition map between them is defined as :<math>\mathbf{R}^n \setminus \{0\} \to \mathbf{R}^n \setminus \{0\}: x \mapsto x/\|x\|^2.</math> This function is its own inverse and thus can be used in both directions. As the transition map is a [[smooth function]], this atlas defines a smooth manifold. In the case ''n'' = 1, the example simplifies to the circle example given earlier. === Identifying points of a manifold === It is possible to define different points of a manifold to be same. This can be visualized as gluing these points together in a single point, forming a [[quotient space]]. There is, however, no reason to expect such quotient spaces to be manifolds. Among the possible quotient spaces that are not necessarily manifolds, [[orbifold]]s and [[CW complex]]es are considered to be relatively [[well-behaved]]. One method of identifying points (gluing them together) is through a right (or left) action of a [[group (mathematics)|group]], which [[group action|acts]] on the manifold. Two points are identified if one is moved onto the other by some group element. If ''M'' is the manifold and ''G'' is the group, the resulting quotient space is denoted by ''M'' / ''G'' (or ''G'' \ ''M''). Manifolds which can be constructed by identifying points include [[torus#topology|tori]] and [[real projective space]]s (starting with a plane and a sphere, respectively). === Cartesian products === The [[Cartesian product]] of manifolds is also a manifold. Not every manifold can be written as a product of other manifolds. The dimension of the product manifold is the sum of the dimensions of its factors. Its topology is the [[product topology]], and a Cartesian product of charts is a chart for the product manifold. Thus, an atlas for the product manifold can be constructed using atlases for its factors. If these atlases define a differential structure on the factors, the corresponding atlas defines a differential structure on the product manifold. The same is true for any other structure defined on the factors. If one of the factors has a boundary, the product manifold also has a boundary. Cartesian products may be used to construct tori and finite [[cylinder (geometry)|cylinder]]s, for example, as '''S'''<sup>1</sup>&nbsp;&times;&nbsp;'''S'''<sup>1</sup> and '''S'''<sup>1</sup>&nbsp;&times;&nbsp;[0,&nbsp;1], respectively. [[Image:Red cylinder.svg|right|thumb|A finite cylinder is a manifold with boundary.]] === Manifold with boundary ===<!-- [[Manifold with boundary]] links here --> A ''manifold with boundary'' is a manifold with an edge. For example a sheet of paper with rounded corners is a [[2-manifold]] with a 1-dimensional boundary. The edge of an ''n''-manifold is an (''n''-1)-manifold. A [[disk (mathematics)|disk]] (circle plus interior) is a 2-manifold with boundary. Its boundary is a circle, a [[1-manifold]]. A [[ball (mathematics)|ball]] (sphere plus interior) is a 3-manifold with boundary. Its boundary is a sphere, a 2-manifold. (See also [[Boundary (topology)]]). In technical language, a manifold with boundary is a space containing both interior points and boundary points. Every interior point has a neighborhood homeomorphic to the open ''n''-ball {(x<sub>1</sub>, x<sub>2</sub>, …, x<sub>n</sub>) | Σ x<sub>i</sub><sup>2</sup> < 1}. Every boundary point has a neighborhood homeomorphic to the "half" ''n''-ball {(x<sub>1</sub>, x<sub>2</sub>, …, x<sub>n</sub>) | Σ x<sub>i</sub><sup>2</sup> < 1 and x<sub>1</sub> ≥ 0}. The homeomorphism must send the boundary point to a point with x<sub>1</sub> = 0. === Gluing along boundaries === Two manifolds with boundaries can be glued together along a boundary. If this is done the right way, the result is also a manifold. Similarly, two boundaries of a single manifold can be glued together. Formally, the gluing is defined by a bijection between the two boundaries. Two points are identified when they are mapped onto each other. For a topological manifold this bijection should be a homeomorphism, otherwise the result will not be a topological manifold. Similarly for a differentiable manifold it has to be a diffeomorphism. For other manifolds other structures should be preserved. A finite cylinder may be constructed as a manifold by starting with a strip [0, 1]&nbsp;&times;&nbsp;[0, 1] and gluing a pair of opposite edges on the boundary by a suitable diffeomorphism. A [[projective plane]] may be obtained by gluing a sphere with a hole in it to a [[Möbius strip]] along their respective circular boundaries. ==Classes of manifolds== {{details|Categories of manifolds}} === Topological manifolds === {{details|topological manifold}} The simplest kind of manifold to define is the topological manifold, which looks locally like some "ordinary" [[Euclidean space]] '''R'''<sup>''n''</sup>. Formally, a topological manifold is a [[topological space]] [[Local homeomorphism|locally homeomorphic]] to a Euclidean space. This means that every point has a neighbourhood for which there exists a [[homeomorphism]] (a [[bijection|bijective]] [[continuous function (topology)|continuous function]] whose inverse is also continuous) mapping that neighbourhood to '''R'''<sup>''n''</sup>. These homeomorphisms are the charts of the manifold. It is to be noted that a ''topological'' manifold looks locally like a euclidean space in a rather weak manner: while for each individual chart it is possible to distinguish differentiable functions or measure distances and angles, merely by virtue of being a topological manifold a space does not have any ''particular'' and ''consistent'' choice of such concepts. In order to discuss such properties for a manifold, one needs to specify further structure and consider [[#Differentiable manifolds|differentiable manifolds]] and [[#Riemannian manifolds|Riemannian manifolds]] discussed below. In particular, a same underlying topological manifold can have several mutually incompatible classes of differentiable functions and an infinite number of ways to specify distances and angles. Usually additional technical assumptions on the topological space are made to exclude pathological cases. It is customary to require that the space be [[Hausdorff space|Hausdorff]] and [[second-countable space|second countable]]. The ''dimension'' of the manifold at a certain point is the dimension of the Euclidean space that the charts at that point map to (number ''n'' in the definition). All points in a [[connected space|connected]] manifold have the same dimension. Some authors require that all charts of a topological manifold map to Euclidean spaces of same dimension. In that case every topological manifold has a topological invariant, its dimension. Other authors allow disjoint unions of topological manifolds with differing dimensions to be called manifolds. === Differentiable manifolds === {{details|differentiable manifold}} For most applications a special kind of topological manifold, a '''differentiable manifold''', is used. If the local charts on a manifold are compatible in a certain sense, one can define directions, tangent spaces, and differentiable functions on that manifold. In particular it is possible to use [[calculus]] on a differentiable manifold. Each point of an ''n''-dimensional differentiable manifold has a [[tangent space]]. This is an ''n''-dimensional Euclidean space consisting of the [[tangent vectors]] of the curves through the point. Two important classes of differentiable manifolds are '''smooth''' and '''analytic manifolds'''. For smooth manifolds the transition maps are [[smooth function|smooth]], that is infinitely differentiable. Analytic manifolds are smooth manifolds with the additional condition that the transition maps are [[analytic function|analytic]] (they can be expressed as power series, which are essentially polynomials of infinite degree). The sphere can be given analytic structure, as can most familiar curves and surfaces. A [[rectifiable set]] generalizes the idea of a piecewise smooth or [[rectifiable curve]] to higher dimensions; however, rectifiable sets are not in general manifolds. === Riemannian manifolds === {{details|Riemannian manifolds}} To measure distances and angles on manifolds, the manifold must be Riemannian. A '''Riemannian manifold''' is a differentiable manifold in which each [[tangent space]] is equipped with an [[Inner product space|inner product]] 〈⋅,⋅〉 in a manner which varies smoothly from point to point. Given two tangent vectors '''u''' and '''v''', the inner product 〈'''u''','''v'''〉 gives a real number. The [[dot product|dot]] (or scalar) product is a typical example of an inner product. This allows one to define various notions such as [[length]], [[angle]]s, [[area]]s (or [[volume]]s), [[curvature]], [[gradient]]s of functions and [[divergence]] of [[vector field]]s. All differentiable manifolds (of constant dimension) can be given the structure of a Riemannian manifold. The Euclidean space itself carries a natural structure of Riemannian manifold (the tangent spaces are naturally identified with the Euclidean space itself and carry the standard scalar product of the space). Many familiar curves and surfaces, including for example all ''n''-spheres, are specified as subspaces of a Euclidean space and inherit a metric from their embedding in it. === Finsler manifolds === {{details|Finsler manifold}} A '''Finsler manifold''' allows the definition of distance, but not of angle; it is an analytic manifold in which each [[tangent space]] is equipped with a [[normed space|norm]], ||·||, in a manner which varies smoothly from point to point. This norm can be extended to a [[Metric (mathematics)|metric]], defining the length of a curve; but it cannot in general be used to define an inner product. Any Riemannian manifold is a Finsler manifold. === Lie groups === {{details|Lie group}} '''Lie groups''', named after [[Sophus Lie]], are differentiable manifolds that carry also the structure of a [[group (mathematics)|group]] which is such that the group operations are defined by smooth maps. A Euclidean vector space with the group operation of vector addition is an example of a non-compact Lie group. A simple example of a [[compact space|compact]] Lie group is the circle: the group operation is simply rotation. This group, known as U(1), can be also characterised as the group of [[complex number]]s of [[Absolute value|modulus]] 1 with multiplication as the group operation. Other examples of Lie groups include special groups of [[Matrix (mathematics)|matrices]], which are all subgroups of the [[general linear group]], the group of ''n'' by ''n'' matrices with non-zero determinant. If the matrix entries are [[real number]]s, this will be an ''n''<sup>2</sup>-dimensional disconnected manifold. The [[orthogonal group]]s, the [[symmetry group]]s of the [[sphere]] and [[hypersphere]]s, are ''n''(''n''-1)/2 dimensional manifolds, where ''n''-1 is the dimension of the sphere. Further examples can be found in the [[table of Lie groups]]. === Other types of manifolds === * A '''[[complex manifold]]''' is a manifold modeled on '''C'''<sup>''n''</sup> with [[holomorphic]] transition functions on [[chart (topology)|chart]] overlaps. These manifolds are the basic objects of study in complex geometry. A one-complex-dimensional manifold is called a [[Riemann surface]]. Note that an ''n''-dimensional complex manifold has dimension 2''n'' as a real differentiable manifold. * A '''[[CR manifold]]''' is a manifold modeled on boundaries of domains in '''C'''<sup>n</sup>. * '''Infinite dimensional manifolds''': to allow for infinite dimensions, one may consider [[Banach manifold]]s which are locally homeomorphic to [[Banach space]]s. Similarly, Fréchet manifolds are locally homeomorphic to [[Fréchet space]]s. * A '''[[symplectic manifold]]''' is a kind of manifold which is used to represent the phase spaces in [[classical mechanics]]. They are endowed with a [[differential form|2-form]] that defines the [[Poisson bracket]]. A closely related type of manifold is a [[Contact geometry|contact manifold]]. ==Classification and invariants== {{details|Classification of manifolds}} Different notions of manifolds have different notions of classification and invariant; in this section we focus on smooth closed manifolds. The classification of smooth closed manifolds is well-understood ''in principle'', except in [[4-manifold|dimension 4]]: in low dimensions (2 and 3) it is geometric, via the [[uniformization theorem]] and the [[Solution of the Poincaré conjecture]], and in high dimension (5 and above) it is algebraic, via [[surgery theory]]. This is a classification in principle: the general question of whether two smooth manifolds are diffeomorphic is [[classification of manifolds#Computability|not computable in general]]. Further, specific computations remain difficult, and there are many open questions. Orientable surfaces can be visualized, and their diffeomorphism classes enumerated, by genus. Given two orientable surfaces, one can determine if they are diffeomorphic by computing their respective genera and comparing: they are diffeomorphic if and only if the genera are equal, so the genus forms a [[complete set of invariants]]. This is much harder in higher dimensions: higher dimensional manifolds cannot be directly visualized (though visual intuition is useful in understanding them), nor can their diffeomorphism classes be enumerated, nor can one in general determine if two different descriptions of a higher-dimensional manifold refer to the same object. However, one can determine if two manifolds are ''different'' if there is some intrinsic characteristic that differentiates them. Such criteria are commonly referred to as '''[[invariant (mathematics)|invariants]]''', because, while they may be defined in terms of some presentation (such as the genus in terms of a triangulation), they are the same relative to all possible descriptions of a particular manifold: they are ''invariant'' under different descriptions. Naively, one could hope to develop an arsenal of invariant criteria that would definitively classify all manifolds up to isomorphism. Unfortunately, it is known that for manifolds of dimension 4 and higher, [[classification of manifolds#Computability|no program exists]] that can decide whether two manifolds are diffeomorphic. Smooth manifolds have [[Classification of manifolds#Enumeration_versus_invariants|a rich set of invariants]], coming from [[point-set topology]], classic [[algebraic topology]], and [[geometric topology]]. The most familiar invariants, which are visible for surfaces, are [[orientability]] (a normal invariant, also detected by [[singular homology|homology]]) and [[genus (mathematics)|genus]] (a homological invariant). Smooth closed manifolds have no local invariants (other than dimension), though geometric manifolds have local invariants, notably the [[curvature of Riemannian manifolds|curvature of a Riemannian manifold]] and the [[torsion (differential geometry)|torsion]] of a manifold equipped with an [[affine connection]]. This distinction between no local invariants and local invariants is a common way to distinguish between [[Geometry and topology#Local_versus_global_structure|geometry and topology]]. All invariants of a smooth closed manifold are thus global. [[Algebraic topology]] is a source of a number of important global invariant properties. Some key criteria include the ''[[simply connected]]'' property and orientability (see below). Indeed several branches of mathematics, such as [[homology (mathematics)|homology]] and [[homotopy]] theory, and the theory of [[characteristic classes]] were founded in order to study invariant properties of manifolds. ==Examples of surfaces== ===Orientability=== In dimensions two and higher, a simple but important invariant criterion is the question of whether a manifold admits a meaningful orientation. Consider a topological manifold with charts mapping to '''R'''<sup>''n''</sup>. Given an [[basis (linear algebra)#Ordered basis|ordered basis]] for '''R'''<sup>''n''</sup>, a chart causes its piece of the manifold to itself acquire a sense of ordering, which in 3-dimensions can be viewed as either right-handed or left-handed. Overlapping charts are not required to agree in their sense of ordering, which gives manifolds an important freedom. For some manifolds, like the sphere, charts can be chosen so that overlapping regions agree on their "handedness"; these are ''[[Orientability|orientable]]'' manifolds. For others, this is impossible. The latter possibility is easy to overlook, because any closed surface embedded (without self-intersection) in three-dimensional space is orientable. Some illustrative examples of non-orientable manifolds include: (1) the [[Möbius strip]], which is a manifold with boundary, (2) the [[Klein bottle]], which must intersect itself in 3-space, and (3) the [[real projective plane]], which arises naturally in [[geometry]]. <!-- see http://planetmath.org/encyclopedia/Orientation2.html --> [[Image:MobiusStrip-01.png|right|thumb|150px|Möbius strip]] ====Möbius strip==== Begin with an infinite circular cylinder standing vertically, a manifold without boundary. Slice across it high and low to produce two circular boundaries, and the cylindrical strip between them. This is an orientable manifold with boundary, upon which "surgery" will be performed. Slice the strip open, so that it could unroll to become a rectangle, but keep a grasp on the cut ends. Twist one end 180°, making the inner surface face out, and glue the ends back together seamlessly. This results in a strip with a permanent half-twist: the [[Möbius strip]]. Its boundary is no longer a pair of circles, but (topologically) a single circle; and what was once its "inside" has merged with its "outside", so that it now has only a ''single'' side. ==== Klein bottle ==== [[Image:KleinBottle-01.png|thumb|120px|right|The Klein bottle immersed in three-dimensional space.]] Take two Möbius strips; each has a single loop as a boundary. Straighten out those loops into circles, and let the strips distort into [[cross-cap]]s. Gluing the circles together will produce a new, closed manifold without boundary, the [[Klein bottle]]. Closing the surface does nothing to improve the lack of orientability, it merely removes the boundary. Thus, the Klein bottle is a closed surface with no distinction between inside and outside. Note that in three-dimensional space, a Klein bottle's surface must pass through itself. Building a Klein bottle which is not self-intersecting requires four or more dimensions of space. ==== Real projective plane ==== Begin with a sphere centered on the origin. Every line through the origin pierces the sphere in two opposite points called ''antipodes''. Although there is no way to do so physically, it is possible to mathematically merge each antipode pair into a single point. The closed surface so produced is the [[real projective plane]], yet another non-orientable surface. It has a number of equivalent descriptions and constructions, but this route explains its name: all the points on any given line through the origin project to the same "point" on this "plane". ===Genus and the Euler characteristic=== For two dimensional manifolds a key invariant property is the [[genus (mathematics)|genus]], or the "number of handles" present in a surface. A torus is a sphere with one handle, a double torus is a sphere with two handles, and so on. Indeed it is possible to fully characterize compact, two-dimensional manifolds on the basis of genus and orientability. In higher-dimensional manifolds genus is replaced by the notion of [[Euler characteristic]]. == Generalizations of manifolds == * '''Orbifolds''': An [[orbifold]] is a generalization of manifold allowing for certain kinds of "[[mathematical singularity|singularities]]" in the topology. Roughly speaking, it is a space which locally looks like the quotients of some simple space (''e.g.'' [[Euclidean space]]) by the [[group action|action]]s of various [[finite group]]s. The singularities correspond to fixed points of the group actions, and the actions must be compatible in a certain sense. * '''Algebraic varieties and schemes''': [[Non-singular]] algebraic varieties over the real or complex numbers are manifolds. One generalizes this first by allowing singularities, secondly by allowing different fields, and thirdly by emulating the patching construction of manifolds: just as a manifold is glued together from open subset of Euclidean space, an [[algebraic variety]] is glued together from affine algebraic varieties, which are zero sets of polynomials over algebraically closed fields. [[Scheme (mathematics)|Schemes]] are likewise glued together from affine schemes, which are a generalization of algebraic varieties. Both are related to manifolds, but are constructed algebraically using [[sheaf (mathematics)|sheaves]] instead of atlases. :Because of [[Mathematical singularity|singular point]]s, a variety is in general not a manifold, though linguistically the French ''variété'', German ''Mannigfaltigkeit'' and English ''manifold'' are largely [[synonymous]]. In French an algebraic variety is called ''une [[:fr:variété algébrique|variété algébrique]]'' (an ''algebraic variety''), while a smooth manifold is called ''une [[:fr:variété différentielle|variété différentielle]]'' (a ''differential variety''). * '''CW-complexes''': A [[CW complex]] is a topological space formed by gluing disks of different dimensionality together. In general the resulting space is singular, and hence not a manifold. However, they are of central interest in [[algebraic topology]], especially in [[homotopy theory]], as they are easy to compute with and singularities are not a concern. == Notes == <references/> == See also == * [[3-manifold]] * [[4-manifold]] * [[5-manifold]] * [[List of manifolds]] * [[Surface]] (2-manifold) == References == * Freedman, Michael H., and Quinn, Frank (1990) ''Topology of 4-Manifolds''. Princeton University Press. ISBN 0-691-08577-3. * Guillemin, Victor and Pollack, Alan (1974) ''Differential Topology''. Prentice-Hall. ISBN 0-13-212605-2. Inspired by Milnor and commonly used in undergraduate courses. * Hempel, John (1976) ''3-Manifolds''. Princeton University Press. ISBN 0-8218-3695-1. * Hirsch, Morris, (1997) ''Differential Topology''. Springer Verlag. ISBN 0-387-90148-5. The most complete account, with historical insights and excellent, but difficult, problems. The standard reference for those wishing to have a deep understanding of the subject. * Kirby, Robion C. and Siebenmann, Laurence C. (1977) ''Foundational Essays on Topological Manifolds. Smoothings, and Triangulations''. Princeton University Press. ISBN 0-691-08190-5. A detailed study of the [[category theory|category]] of topological manifolds. * Lee, John M. (2000) ''Introduction to Topological Manifolds''. Springer-Verlag. ISBN 0-387-98759-2. * ------ (2003) ''Introduction to Smooth Manifolds''. Springer-Verlag. ISBN 0-387-95495-3. * Massey, William S. (1977) ''Algebraic Topology: An Introduction''. Springer-Verlag. ISBN 0-387-90271-6. * [[John Milnor|Milnor, John]] (1997) ''Topology from the Differentiable Viewpoint''. Princeton University Press. ISBN 0-691-04833-9. * [[Michael Spivak|Spivak, Michael]] (1965) ''Calculus on Manifolds: A Modern Approach to Classical Theorems of Advanced Calculus''. HarperCollins Publishers. ISBN 0-8053-9021-9. The standard graduate text. * Munkres, James R. (2000) ''Topology''. Prentice Hall. ISBN 0-13-181629-2. * Neuwirth, L. P., ed. (1975) ''Knots, Groups, and 3-Manifolds. Papers Dedicated to the Memory of R. H. Fox''. Princeton University Press. ISBN 978-0-691-08170-0. * [[Bernhard Riemann|Riemann, Bernhard]], ''Gesammelte mathematische Werke und wissenschaftlicher Nachlass'', Sändig Reprint. ISBN 3-253-03059-8. **''[http://www.maths.tcd.ie/pub/HistMath/People/Riemann/Grund/ Grundlagen für eine allgemeine Theorie der Functionen einer veränderlichen complexen Grösse.]'' The 1851 doctoral thesis in which "manifold" (''Mannigfaltigkeit'') first appears. **''[http://www.maths.tcd.ie/pub/HistMath/People/Riemann/Geom/ Ueber die Hypothesen, welche der Geometrie zu Grunde liegen.]'' The 1854 Göttingen inaugural lecture (''Habilitationsschrift''). 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