Introductory examples
In the words of Michael Atiyah, The following list of examples shows the common features of many dualities, but also indicates that the precise meaning of duality may vary from case to case.Complement of a subset
A simple, maybe the most simple, duality arises from considering subsets of a fixed set . To any subset , the complement consists of all those elements in that are not contained in . It is again a subset of . Taking the complement has the following properties: * Applying it twice gives back the original set, i.e., . This is referred to by saying that the operation of taking the complement is an '' involution''. * An inclusion of sets is turned into an inclusion in the ''opposite'' direction . * Given two subsets and of , is contained in if and only if is contained in . This duality appears in topology as a duality betweenDual cone
A duality in geometry is provided by the dual cone construction. Given a set of points in the plane (or more generally points in the dual cone is defined as the set consisting of those points satisfying for all points in , as illustrated in the diagram. Unlike for the complement of sets mentioned above, it is not in general true that applying the dual cone construction twice gives back the original set . Instead, is the smallest cone containing which may be bigger than . Therefore this duality is weaker than the one above, in that * Applying the operation twice gives back a possibly bigger set: for all , is contained in . (For some , namely the cones, the two are actually equal.) The other two properties carry over without change: * It is still true that an inclusion is turned into an inclusion in the opposite direction (). * Given two subsets and of the plane, is contained in if and only if is contained in .Dual vector space
A very important example of a duality arises in linear algebra by associating to any vector space its dual vector space . Its elements are the linear functionals , where is the field over which is defined. The three properties of the dual cone carry over to this type of duality by replacing subsets of by vector space and inclusions of such subsets by linear maps. That is: * Applying the operation of taking the dual vector space twice gives another vector space . There is always a map . For some , namely precisely the finite-dimensional vector spaces, this map is an isomorphism. * A linear map gives rise to a map in the opposite direction (). * Given two vector spaces and , the maps from to correspond to the maps from to . A particular feature of this duality is that and are isomorphic for certain objects, namely finite-dimensional vector spaces. However, this is in a sense a lucky coincidence, for giving such an isomorphism requires a certain choice, for example the choice of a basis of . This is also true in the case if is a Hilbert space, ''via'' the Riesz representation theorem.Galois theory
In all the dualities discussed before, the dual of an object is of the same kind as the object itself. For example, the dual of a vector space is again a vector space. Many duality statements are not of this kind. Instead, such dualities reveal a close relation between objects of seemingly different nature. One example of such a more general duality is from Galois theory. For a fixed Galois extension , one may associate the Galois group to any intermediate field (i.e., ). This group is a subgroup of the Galois group . Conversely, to any such subgroup there is the fixed field consisting of elements fixed by the elements in . Compared to the above, this duality has the following features: * An extension of intermediate fields gives rise to an inclusion of Galois groups in the opposite direction: . * Associating to and to are inverse to each other. This is the content of theOrder-reversing dualities
Given a poset (short for partially ordered set; i.e., a set that has a notion of ordering but in which two elements cannot necessarily be placed in order relative to each other), the dual poset comprises the same ground set but the converse relation. Familiar examples of dual partial orders include * the subset and superset relations and on any collection of sets, such as the subsets of a fixed set . This gives rise to the first example of a duality mentioned above. * the ''divides'' and ''multiple-of'' relations on the integers. * the ''descendant-of'' and ''ancestor-of'' relations on the set of humans. A ''duality transform'' is an involutive antiautomorphism of a partially ordered set , that is, an order-reversing involution . In several important cases these simple properties determine the transform uniquely up to some simple symmetries. For example, if , are two duality transforms then their composition is an order automorphism of ; thus, any two duality transforms differ only by an order automorphism. For example, all order automorphisms of a power set are induced by permutations of . A concept defined for a partial order will correspond to a ''dual concept'' on the dual poset . For instance, a minimal element of will be a maximal element of : minimality and maximality are dual concepts in order theory. Other pairs of dual concepts are upper and lower bounds, lower sets and upper sets, and ideals andDimension-reversing dualities
There are many distinct but interrelated dualities in which geometric or topological objects correspond to other objects of the same type, but with a reversal of the dimensions of the features of the objects. A classical example of this is the duality of the Platonic solids, in which the cube and the octahedron form a dual pair, the dodecahedron and the icosahedron form a dual pair, and the tetrahedron is self-dual. The dual polyhedron of any of these polyhedra may be formed as the convex hull of the center points of each face of the primal polyhedron, so the vertices of the dual correspond one-for-one with the faces of the primal. Similarly, each edge of the dual corresponds to an edge of the primal, and each face of the dual corresponds to a vertex of the primal. These correspondences are incidence-preserving: if two parts of the primal polyhedron touch each other, so do the corresponding two parts of the dual polyhedron. More generally, using the concept of polar reciprocation, any convex polyhedron, or more generally any convex polytope, corresponds to a dual polyhedron or dual polytope, with an -dimensional feature of an -dimensional polytope corresponding to an -dimensional feature of the dual polytope. The incidence-preserving nature of the duality is reflected in the fact that the face lattices of the primal and dual polyhedra or polytopes are themselves order-theoretic duals. Duality of polytopes and order-theoretic duality are both involutions: the dual polytope of the dual polytope of any polytope is the original polytope, and reversing all order-relations twice returns to the original order. Choosing a different center of polarity leads to geometrically different dual polytopes, but all have the same combinatorial structure. From any three-dimensional polyhedron, one can form a planar graph, the graph of its vertices and edges. The dual polyhedron has a dual graph, a graph with one vertex for each face of the polyhedron and with one edge for every two adjacent faces. The same concept of planar graph duality may be generalized to graphs that are drawn in the plane but that do not come from a three-dimensional polyhedron, or more generally to graph embeddings on surfaces of higher genus: one may draw a dual graph by placing one vertex within each region bounded by a cycle of edges in the embedding, and drawing an edge connecting any two regions that share a boundary edge. An important example of this type comes from computational geometry: the duality for any finite set of points in the plane between the Delaunay triangulation of and the Voronoi diagram of . As with dual polyhedra and dual polytopes, the duality of graphs on surfaces is a dimension-reversing involution: each vertex in the primal embedded graph corresponds to a region of the dual embedding, each edge in the primal is crossed by an edge in the dual, and each region of the primal corresponds to a vertex of the dual. The dual graph depends on how the primal graph is embedded: different planar embeddings of a single graph may lead to different dual graphs.Duality in logic and set theory
In logic, functions or relations and are considered dual if , where ¬ is logical negation. The basic duality of this type is the duality of the ∃ and ∀ quantifiers in classical logic. These are dual because and are equivalent for all predicates in classical logic: if there exists an for which fails to hold, then it is false that holds for all (but the converse does not hold constructively). From this fundamental logical duality follow several others: * A formula is said to be '' satisfiable'' in a certain model if there are assignments to its free variables that render it true; it is ''valid'' if ''every'' assignment to its free variables makes it true. Satisfiability and validity are dual because the invalid formulas are precisely those whose negations are satisfiable, and the unsatisfiable formulas are those whose negations are valid. This can be viewed as a special case of the previous item, with the quantifiers ranging over interpretations. * In classical logic, the and operators are dual in this sense, because and are equivalent. This means that for every theorem of classical logic there is an equivalent dual theorem. De Morgan's laws are examples. More generally, . The left side is true if and only if , and the right side if and only if ¬∃''i''.''x''''i''. * In modal logic, means that the proposition is "necessarily" true, and that is "possibly" true. Most interpretations of modal logic assign dual meanings to these two operators. For example in Kripke semantics, " is possibly true" means "there exists some world such that is true in ", while " is necessarily true" means "for all worlds , is true in ". The duality of and then follows from the analogous duality of and . Other dual modal operators behave similarly. For example, temporal logic has operators denoting "will be true at some time in the future" and "will be true at all times in the future" which are similarly dual. Other analogous dualities follow from these: * Set-theoretic union and intersection are dual under the set complement operator . That is, , and more generally, . This follows from the duality of and : an element is a member of if and only if , and is a member of if and only if .Dual objects
A group of dualities can be described by endowing, for any mathematical object , the set of morphisms into some fixed object , with a structure similar to that of . This is sometimes calledDual vector spaces revisited
The construction of the dual vector space mentioned in the introduction is an example of such a duality. Indeed, the set of morphisms, i.e., linear maps, forms a vector space in its own right. The map mentioned above is always injective. It is surjective, and therefore an isomorphism, if and only if the dimension of is finite. This fact characterizes finite-dimensional vector spaces without referring to a basis.Isomorphisms of and and inner product spaces
A vector space is isomorphic to precisely if is finite-dimensional. In this case, such an isomorphism is equivalent to a non-degenerate bilinear form In this case is called an inner product space. For example, if is the field of real or complex numbers, anyDuality in projective geometry
In some projective planes, it is possible to find geometric transformations that map each point of the projective plane to a line, and each line of the projective plane to a point, in an incidence-preserving way. For such planes there arises a general principle of duality in projective planes: given any theorem in such a plane projective geometry, exchanging the terms "point" and "line" everywhere results in a new, equally valid theorem. A simple example is that the statement "two points determine a unique line, the line passing through these points" has the dual statement that "two lines determine a unique point, theTopological vector spaces and Hilbert spaces
In the realm of topological vector spaces, a similar construction exists, replacing the dual by the topological dual vector space. There are several notions of topological dual space, and each of them gives rise to a certain concept of duality. A topological vector space that is canonically isomorphic to its bidual is called a reflexive space: Examples: * As in the finite-dimensional case, on each Hilbert space its inner product defines a map which is a bijection due to the Riesz representation theorem. As a corollary, every Hilbert space is aFurther dual objects
TheDual categories
Opposite category and adjoint functors
In another group of dualities, the objects of one theory are translated into objects of another theory and the maps between objects in the first theory are translated into morphisms in the second theory, but with direction reversed. Using the parlance of category theory, this amounts to a contravariant functor between two categories and : which for any two objects ''X'' and ''Y'' of ''C'' gives a map That functor may or may not be anSpaces and functions
Gelfand duality is a duality between commutative C*-algebras ''A'' andGalois connections
In a number of situations, the two categories which are dual to each other are actually arising from partially ordered sets, i.e., there is some notion of an object "being smaller" than another one. A duality that respects the orderings in question is known as a Galois connection. An example is the standard duality in Galois theory mentioned in the introduction: a bigger field extension corresponds—under the mapping that assigns to any extension ''L'' ⊃ ''K'' (inside some fixed bigger field Ω) the Galois group Gal (Ω / ''L'') —to a smaller group. The collection of all open subsets of a topological space ''X'' forms a complete Heyting algebra. There is a duality, known as Stone duality, connecting sober spaces and spatial locale (mathematics), locales. * Birkhoff's representation theorem relating distributive lattices and partial ordersPontryagin duality
Pontryagin duality gives a duality on the category of locally compact abelian groups: given any such group ''G'', the character group :χ(''G'') = Hom (''G'', ''S''1) given by continuous group homomorphisms from ''G'' to the circle group ''S''1 can be endowed with the compact-open topology. Pontryagin duality states that the character group is again locally compact abelian and that :''G'' ≅ χ(χ(''G'')). Moreover, discrete groups correspond to compact group, compact abelian groups; finite groups correspond to finite groups. On the one hand, Pontryagin is a special case of Gelfand duality. On the other hand, it is the conceptual reason of Fourier analysis, see below.Analytic dualities
In mathematical analysis, analysis, problems are frequently solved by passing to the dual description of functions and operators. Fourier transform switches between functions on a vector space and its dual: and conversely If ''f'' is an Lebesgue integration, ''L''2-function on R or R''N'', say, then so is and . Moreover, the transform interchanges operations of multiplication and convolution on the corresponding function spaces. A conceptual explanation of the Fourier transform is obtained by the aforementioned Pontryagin duality, applied to the locally compact groups R (or R''N'' etc.): any character of R is given by ξ ↦ ''e''−2''πixξ''. The dualizing character of Fourier transform has many other manifestations, for example, in alternative descriptions of quantum mechanics, quantum mechanical systems in terms of coordinate and momentum representations. * Laplace transform is similar to Fourier transform and interchanges linear operator, operators of multiplication by polynomials with constant coefficient linear differential operators. * Legendre transformation is an important analytic duality which switches between velocity, velocities in Lagrangian mechanics and momentum, momenta in Hamiltonian mechanics.Homology and cohomology
Theorems showing that certain objects of interest are the dual spaces (in the sense of linear algebra) of other objects of interest are often called ''dualities''. Many of these dualities are given by a bilinear function, bilinear pairing of two ''K''-vector spaces :''A'' ⊗ ''B'' → ''K''. For perfect pairings, there is, therefore, an isomorphism of ''A'' to the dual vector space, dual of ''B''.Poincaré duality
Poincaré duality of a smooth compact complex manifold ''X'' is given by a pairing of singular cohomology with C-coefficients (equivalently, sheaf cohomology of the constant sheaf C) :H''i''(X) ⊗ H2''n''−''i''(X) → C, where ''n'' is the (complex) dimension of ''X''. Poincaré duality can also be expressed as a relation of singular homology and de Rham cohomology, by asserting that the map : (integrating a differential ''k''-form over an 2''n''−''k''-(real) -dimensional cycle) is a perfect pairing. Poincaré duality also reverses dimensions; it corresponds to the fact that, if a topological manifold is represented as a cell complex, then the dual of the complex (a higher-dimensional generalization of the planar graph dual) represents the same manifold. In Poincaré duality, this homeomorphism is reflected in an isomorphism of the ''k''th homology (mathematics), homology group and the (''n'' − ''k'')th cohomology group.Duality in algebraic and arithmetic geometry
The same duality pattern holds for a smooth projective variety over a separably closed field, using l-adic cohomology with Qℓ-coefficients instead. This is further generalized to possibly singular variety, singular varieties, using intersection cohomology instead, a duality called Verdier duality. Serre duality or coherent duality are similar to the statements above, but applies to cohomology of coherent sheaves instead. With increasing level of generality, it turns out, an increasing amount of technical background is helpful or necessary to understand these theorems: the modern formulation of these dualities can be done using derived category, derived categories and certain image functors for sheaves, direct and inverse image functors of sheaves (with respect to the classical analytical topology on manifolds for Poincaré duality, l-adic sheaves and the étale topology in the second case, and with respect to coherent sheaves for coherent duality). Yet another group of similar duality statements is encountered in arithmetics: étale cohomology of finite field, finite, local field, local and global fields (also known as Galois cohomology, since étale cohomology over a field is equivalent to group cohomology of the (absolute) Galois group of the field) admit similar pairings. The absolute Galois group ''G''(F''q'') of a finite field, for example, is isomorphic to , the profinite completion of Z, the integers. Therefore, the perfect pairing (for any G-module, ''G''-module ''M'') :H''n''(''G'', ''M'') × H1−''n'' (''G'', Hom (''M'', Q/Z)) → Q/Z is a direct consequence of Pontryagin duality of finite groups. For local and global fields, similar statements exist (Local Tate duality, local duality and global or Poitou–Tate duality).;See also
* Adjoint functor * Autonomous category * Dual abelian variety * Dual basis * Dual (category theory) * Dual code * Duality (electrical engineering) * Duality (optimization) * Dualizing module * Dualizing sheaf * Dual lattice * Dual norm * Dual numbers, a certain associative algebra; the term "dual" here is synonymous with ''double'', and is unrelated to the notions given above. * Dual system * Koszul duality * Langlands dual * Linear programming#Duality * List of dualities * Matlis duality * Petrie polygon#Petrie dual, Petrie duality * Pontryagin duality * S-duality * T-duality, Homological mirror symmetry, Mirror symmetryNotes
References
Duality in general
* Atiyah, Michael (2007)Duality in algebraic topology
*James C. Becker and Daniel Henry GottliebSpecific dualities
* . Als