From Wikipedia, the free encyclopedia

Please help improve this article or section by expanding it. Further information might be found on the talk page or at requests for expansion. (April 2007)

In mathematics, duality has numerous meanings. Generally speaking, dualities translate concepts, theorems or mathematical structures into other concepts, theorems or structures, in a one-to-one fashion. Duality is characteristically an involution operation: if the dual of A is B, then the dual of B is A. As involutions sometimes have fixed points, the dual of A is sometimes A itself.

In one group of dualities, the concepts and theorems of a certain mathematical theory are mechanically translated into other concepts and theorems of the same theory. The prototypical example here is the duality in projective geometry: given any theorem in plane projective geometry, exchanging the terms "point" and "line" everywhere results in a new, equally valid theorem. Other examples include:

• dual polyhedron
• dual graph of a planar graph
• dual problem in optimization theory
• De Morgan dual in logic (and its analogues, the dual quantifiers in first-order logic and modal logic)
• duality in order theory

In another group of dualities, the objects of one theory are translated into objects of another theory and the morphisms between objects in the first theory are translated into morphisms in the second theory, but with direction reversed. 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. For the general notion in category theory that underlies these dualities, see opposite category. Examples include:

• dual spaces in linear algebra
• duality between commutative rings and algebraic varieties; compare with noncommutative geometry
• Pontryagin duality, relating certain abelian groups to other abelian groups and the background to Fourier analysis
• Tannaka-Krein duality, a non-commutative analogue of Pontryagin duality
• Stone duality, relating Boolean algebras to certain topological spaces
• duality between submodules and factor modules in algebra
• categorical duality between projective modules and injective modules in homological algebra

In analysis, frequently problems are solved by passing to the dual description of functions and operators.

• Fourier transform switches between functions on a vector space and its dual, and interchanges operations of multiplication and convolution on the corresponding function spaces; its dualizing character has many other manifestations, for example, in alternative descriptions of quantum mechanical systems in terms of coordinate and momentum representations.
• Laplace transform is similar to Fourier transform and interchanges operators of multiplication by polynomials with constant coefficient linear differential operators.
• Legendre transformation is an important analytic duality which switches between velocities in Lagrangian mechanics and momenta in Hamiltonian mechanics.

In Boolean algebra a self dual function is one such that:

If f(a₁, ... , a_n) = ~f(~a₁, ... , ~a_n) for all a₁, ... , a_n {0,1}

Negation is self dual.

Theorems showing that certain objects of interest are the dual spaces (in the sense of linear algebra) of other objects of interest are also often called dualities. Examples:

• Poincaré duality
• Alexander duality
• Serre duality
  • Coherent duality

• Dual (category theory)
• Dual numbers, a certain associative algebra; the term "dual" here is synonymous with double, and is unrelated to the notions given above.
• Dual graph in graph theory
• Hodge dual
• Duality (electrical engineering)
• Lagrange duality
• Dual code