# Duality (mathematics)

In mathematics, a **duality** translates concepts, theorems or mathematical structures into other concepts, theorems or structures, in a one-to-one fashion, often (but not always) by means of an involution operation: if the dual of *A* is *B*, then the dual of *B* is *A*. Such involutions sometimes have fixed points, so that the dual of *A* is *A* itself. For example, Desargues' theorem is **self-dual** in this sense under the *standard duality in projective geometry*.

In mathematical contexts, *duality* has numerous meanings.^{[1]} It has been described as "a very pervasive and important concept in (modern) mathematics"^{[2]} and "an important general theme that has manifestations in almost every area of mathematics".^{[3]}

Many mathematical dualities between objects of two types correspond to pairings, bilinear functions from an object of one type and another object of the second type to some family of scalars. For instance, *linear algebra duality* corresponds in this way to bilinear maps from pairs of vector spaces to scalars, the *duality between distributions and the associated test functions* corresponds to the pairing in which one integrates a distribution against a test function, and *Poincaré duality* corresponds similarly to intersection number, viewed as a pairing between submanifolds of a given manifold.^{[4]}

From a category theory viewpoint, duality can also be seen as a functor, at least in the realm of vector spaces. This functor assigns to each space its dual space, and the pullback construction assigns to each arrow *f*: *V* → *W* its dual *f*^{∗}: *W*^{∗} → *V*^{∗}.

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.

A simple, maybe the most simple, duality arises from considering subsets of a fixed set `S`. To any subset `A` ⊆ `S`, the complement `A`^{c}^{[6]} consists of all those elements in `S` that are not contained in `A`. It is again a subset of `S`. Taking the complement has the following properties:

This duality appears in topology as a duality between open and closed subsets of some fixed topological space `X`: a subset `U` of `X` is closed if and only if its complement in `X` is open. Because of this, many theorems about closed sets are dual to theorems about open sets. For example, any union of open sets is open, so dually, any intersection of closed sets is closed. The interior of a set is the largest open set contained in it, and the closure of the set is the smallest closed set that contains it. Because of the duality, the complement of the interior of any set `U` is equal to the closure of the complement of `U`.

A particular feature of this duality is that `V` and `V`^{*} 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 `V`. This is also true in the case if `V` is a Hilbert space, *via* the Riesz representation theorem.

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 `K` / `F`, one may associate the Galois group Gal(`K`/`E`) to any intermediate field `E` (i.e., `F` ⊆ `E` ⊆ `K`). This group is a subgroup of the Galois group `G` = Gal(`K`/`F`). Conversely, to any such subgroup `H` ⊆ `G` there is the fixed field `K`^{H} consisting of elements fixed by the elements in `H`.

Given a poset `P` = (`X`, ≤) (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 `P`^{d} = (`X`, ≥) comprises the same ground set but the converse relation. Familiar examples of dual partial orders include

A *duality transform* is an involutive antiautomorphism `f` of a partially ordered set `S`, that is, an order-reversing involution `f` : `S` → `S`.^{[8]}^{[9]} In several important cases these simple properties determine the transform uniquely up to some simple symmetries. For example, if `f`_{1}, `f`_{2} are two duality transforms then their composition is an order automorphism of `S`; thus, any two duality transforms differ only by an order automorphism. For example, all order automorphisms of a power set `S` = 2^{R} are induced by permutations of `R`.

A concept defined for a partial order `P` will correspond to a *dual concept* on the dual poset `P`^{d}. For instance, a minimal element of `P` will be a maximal element of `P`^{d}: 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 and filters.

In topology, open sets and closed sets are dual concepts: the complement of an open set is closed, and vice versa. In matroid theory, the family of sets complementary to the independent sets of a given matroid themselves form another matroid, called the dual matroid.

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 `i`-dimensional feature of an `n`-dimensional polytope corresponding to an (`n` − `i` − 1)-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 `S` of points in the plane between the Delaunay triangulation of `S` and the Voronoi diagram of `S`. 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. Matroid duality is an algebraic extension of planar graph duality, in the sense that the dual matroid of the graphic matroid of a planar graph is isomorphic to the graphic matroid of the dual graph.

In logic, functions or relations `A` and `B` are considered dual if `A`(¬`x`) = ¬`B`(`x`), where ¬ is logical negation. The basic duality of this type is the duality of the ∃ and ∀ quantifiers in classical logic. These are dual because ∃`x`.¬`P`(`x`) and ¬∀`x`.`P`(`x`) are equivalent for all predicates `P` in classical logic: if there exists an `x` for which `P` fails to hold, then it is false that `P` holds for all `x` (but the converse does not hold constructively). From this fundamental logical duality follow several others:

A group of dualities can be described by endowing, for any mathematical object `X`, the set of morphisms Hom (`X`, `D`) into some fixed object `D`, with a structure similar to that of `X`. This is sometimes called internal Hom. In general, this yields a true duality only for specific choices of `D`, in which case `X`^{*} = Hom (`X`, `D`) is referred to as the *dual* of `X`. There is always a map from `X` to the *bidual*, that is to say, the dual of the dual,

It assigns to some `x` ∈ `X` the map that associates to any map `f` : `X` → `D` (i.e., an element in Hom(`X`, `D`)) the value `f`(`x`).
Depending on the concrete duality considered and also depending on the object `X`, this map may or may not be an isomorphism.

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 `V` → `V`^{**} mentioned above is always injective. It is surjective, and therefore an isomorphism, if and only if the dimension of `V` is finite. This fact characterizes finite-dimensional vector spaces without referring to a basis.

A vector space *V* is isomorphic to *V*^{∗} precisely if *V* is finite-dimensional. In this case, such an isomorphism is equivalent to a non-degenerate bilinear form

In this case *V* is called an inner product space.
For example, if *K* is the field of real or complex numbers, any positive definite bilinear form gives rise to such an isomorphism. In Riemannian geometry, *V* is taken to be the tangent space of a manifold and such positive bilinear forms are called Riemannian metrics. Their purpose is to measure angles and distances. Thus, duality is a foundational basis of this branch of geometry. Another application of inner product spaces is the Hodge star which provides a correspondence between the elements of the exterior algebra. For an *n*-dimensional vector space, the Hodge star operator maps *k*-forms to (*n* − *k*)-forms. This can be used to formulate Maxwell's equations. In this guise, the duality inherent in the inner product space exchanges the role of magnetic and electric fields.

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.^{[10]} 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.^{[11]} 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, the intersection point of these two lines". For further examples, see Dual theorems.

which is used in the construction of toric varieties.^{[15]} The Pontryagin dual of locally compact topological groups *G* is given by

continuous group homomorphisms with values in the circle (with multiplication of complex numbers as group operation).

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 `C` and `D`:

That functor may or may not be an equivalence of categories. There are various situations, where such a functor is an equivalence between the opposite category `C`^{op} of `C`, and `D`. Using a duality of this type, every statement in the first theory can be translated into a "dual" statement in the second theory, where the direction of all arrows has to be reversed.^{[16]} Therefore, any duality between categories `C` and `D` is formally the same as an equivalence between `C` and `D`^{op} (`C`^{op} and `D`). However, in many circumstances the opposite categories have no inherent meaning, which makes duality an additional, separate concept.^{[17]}

A category that is equivalent to its dual is called *self-dual*. An example of self-dual category is the category of Hilbert spaces.^{[18]}

Many category-theoretic notions come in pairs in the sense that they correspond to each other while considering the opposite category. For example, Cartesian products `Y`_{1} × `Y`_{2} and disjoint unions `Y`_{1} ⊔ `Y`_{2} of sets are dual to each other in the sense that

for any set `X`. This is a particular case of a more general duality phenomenon, under which limits in a category `C` correspond to colimits in the opposite category `C`^{op}; further concrete examples of this are epimorphisms vs. monomorphism, in particular factor modules (or groups etc.) vs. submodules, direct products vs. direct sums (also called coproducts to emphasize the duality aspect). Therefore, in some cases, proofs of certain statements can be halved, using such a duality phenomenon. Further notions displaying related by such a categorical duality are projective and injective modules in homological algebra,^{[19]} fibrations and cofibrations in topology and more generally model categories.^{[20]}

Two functors `F`: `C` → `D` and `G`: `D` → `C` are adjoint if for all objects *c* in *C* and *d* in *D*

in a natural way. Actually, the correspondence of limits and colimits is an example of adjoints, since there is an adjunction

between the colimit functor that assigns to any diagram in `C` indexed by some category `I` its colimit and the diagonal functor that maps any object `c` of `C` to the constant diagram which has `c` at all places. Dually,

Gelfand duality is a duality between commutative C*-algebras *A* and compact Hausdorff spaces *X* is the same: it assigns to *X* the space of continuous functions (which vanish at infinity) from *X* to **C**, the complex numbers. Conversely, the space *X* can be reconstructed from *A* as the spectrum of *A*. Both Gelfand and Pontryagin duality can be deduced in a largely formal, category-theoretic way.^{[21]}

In a similar vein there is a duality in algebraic geometry between commutative rings and affine schemes: to every commutative ring *A* there is an affine spectrum, Spec *A*. Conversely, given an affine scheme *S*, one gets back a ring by taking global sections of the structure sheaf O_{S}. In addition, ring homomorphisms are in one-to-one correspondence with morphisms of affine schemes, thereby there is an equivalence

Affine schemes are the local building blocks of schemes. The previous result therefore tells that the local theory of schemes is the same as commutative algebra, the study of commutative rings.

Noncommutative geometry draws inspiration from Gelfand duality and studies noncommutative C*-algebras as if they were functions on some imagined space. Tannaka–Krein duality is a non-commutative analogue of Pontryagin duality.^{[23]}

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.^{[24]}

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 locales.

Pontryagin duality gives a duality on the category of locally compact abelian groups: given any such group *G*, the character group

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

Moreover, discrete groups correspond to 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.

In 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:

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 pairing of two *K*-vector spaces

For perfect pairings, there is, therefore, an isomorphism of *A* to the dual of *B*.

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**)

where *n* is the (complex) dimension of *X*.^{[26]} 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 group and the (*n* − *k*)th cohomology group.

The same duality pattern holds for a smooth projective variety over a separably closed field, using l-adic cohomology with **Q**_{ℓ}-coefficients instead.^{[27]} This is further generalized to possibly singular varieties, using intersection cohomology instead, a duality called Verdier duality.^{[28]} Serre duality or coherent duality are similar to the statements above, but applies to cohomology of coherent sheaves instead.^{[29]}

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 categories and certain 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).

is a direct consequence of Pontryagin duality of finite groups. For local and global fields, similar statements exist (local duality and global or Poitou–Tate duality).^{[31]}