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mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a partition of unity of a
topological space In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called poin ...
is a
set Set, The Set, SET or SETS may refer to: Science, technology, and mathematics Mathematics *Set (mathematics), a collection of elements *Category of sets, the category whose objects and morphisms are sets and total functions, respectively Electro ...
of
continuous function In mathematics, a continuous function is a function such that a continuous variation (that is a change without jump) of the argument induces a continuous variation of the value of the function. This means that there are no abrupt changes in val ...
s from to the
unit interval In mathematics, the unit interval is the closed interval , that is, the set of all real numbers that are greater than or equal to 0 and less than or equal to 1. It is often denoted ' (capital letter ). In addition to its role in real analysis ...
,1such that for every point x\in X: * there is a
neighbourhood A neighbourhood (British English, Irish English, Australian English and Canadian English) or neighborhood (American English; see spelling differences) is a geographically localised community within a larger city, town, suburb or rural a ...
of where all but a finite number of the functions of are 0, and * the sum of all the function values at is 1, i.e., \sum_ \rho(x) = 1. Partitions of unity are useful because they often allow one to extend local constructions to the whole space. They are also important in the
interpolation In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points. In engineering and science, one often has ...
of data, in
signal processing Signal processing is an electrical engineering subfield that focuses on analyzing, modifying and synthesizing '' signals'', such as sound, images, and scientific measurements. Signal processing techniques are used to optimize transmissions, ...
, and the theory of spline functions.


Existence

The existence of partitions of unity assumes two distinct forms: # Given any
open cover In mathematics, and more particularly in set theory, a cover (or covering) of a set X is a collection of subsets of X whose union is all of X. More formally, if C = \lbrace U_\alpha : \alpha \in A \rbrace is an indexed family of subsets U_\alp ...
\_ of a space, there exists a partition \_ indexed ''over the same set'' such that supp \rho_i \subseteq U_i. Such a partition is said to be subordinate to the open cover \_i. # If the space is locally-compact, given any open cover \_ of a space, there exists a partition \_ indexed over a possibly distinct index set such that each has
compact support In mathematics, the support of a real-valued function f is the subset of the function domain containing the elements which are not mapped to zero. If the domain of f is a topological space, then the support of f is instead defined as the smalle ...
and for each , supp \rho_j \subseteq U_i for some . Thus one chooses either to have the supports indexed by the open cover, or compact supports. If the space is
compact Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to: * Interstate compact * Blood compact, an ancient ritual of the Philippines * Compact government, a type of colonial rule utilized in Britis ...
, then there exist partitions satisfying both requirements. A finite open cover always has a continuous partition of unity subordinated to it, provided the space is locally compact and Hausdorff. Paracompactness of the space is a necessary condition to guarantee the existence of a partition of unity subordinate to any open cover. Depending on the
category Category, plural categories, may refer to: Philosophy and general uses *Categorization, categories in cognitive science, information science and generally * Category of being * ''Categories'' (Aristotle) * Category (Kant) * Categories (Peirce) ...
to which the space belongs, it may also be a sufficient condition. The construction uses
mollifier In mathematics, mollifiers (also known as ''approximations to the identity'') are smooth functions with special properties, used for example in distribution theory to create sequences of smooth functions approximating nonsmooth (generalized) f ...
s (bump functions), which exist in continuous and smooth manifolds, but not in
analytic manifold In mathematics, an analytic manifold, also known as a C^\omega manifold, is a differentiable manifold with analytic transition maps. The term usually refers to real analytic manifolds, although complex manifolds are also analytic. In algebraic ge ...
s. Thus for an open cover of an analytic manifold, an analytic partition of unity subordinate to that open cover generally does not exist. ''See''
analytic continuation In complex analysis, a branch of mathematics, analytic continuation is a technique to extend the domain of definition of a given analytic function. Analytic continuation often succeeds in defining further values of a function, for example in a ...
. If and are partitions of unity for spaces and , respectively, then the set of all pairs \ is a partition of unity for the
cartesian product In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is : A\t ...
space . The tensor product of functions act as (\rho \otimes \tau )(x,y) = \rho(x)\tau(y).


Example

We can construct a partition of unity on S^1 by looking at a chart on the complement of a point p \in S^1 sending S^1 -\ to \mathbb with center q \in S^1. Now, let \Phi be a
bump function In mathematics, a bump function (also called a test function) is a function f: \R^n \to \R on a Euclidean space \R^n which is both smooth (in the sense of having continuous derivatives of all orders) and compactly supported. The set of all bum ...
on \mathbb defined by \Phi(x) = \begin \exp\left(\frac\right) & x \in (-1,1) \\ 0 & \text \end then, both this function and 1 - \Phi can be extended uniquely onto S^1 by setting \Phi(p) = 0. Then, the set \ forms a partition of unity over S^1.


Variant definitions

Sometimes a less restrictive definition is used: the sum of all the function values at a particular point is only required to be positive, rather than 1, for each point in the space. However, given such a set of functions \_^\infty one can obtain a partition of unity in the strict sense by dividing by the sum; the partition becomes \_^\infty where \sigma(x) := \sum_^\infty \psi_i(x), which is well defined since at each point only a finite number of terms are nonzero. Even further, some authors drop the requirement that the supports be locally finite, requiring only that \sum_^\infty \psi_i(x) < \infty for all x.


Applications

A partition of unity can be used to define the integral (with respect to a
volume form In mathematics, a volume form or top-dimensional form is a differential form of degree equal to the differentiable manifold dimension. Thus on a manifold M of dimension n, a volume form is an n-form. It is an element of the space of sections of th ...
) of a function defined over a manifold: One first defines the integral of a function whose support is contained in a single coordinate patch of the manifold; then one uses a partition of unity to define the integral of an arbitrary function; finally one shows that the definition is independent of the chosen partition of unity. A partition of unity can be used to show the existence of a Riemannian metric on an arbitrary manifold. Method of steepest descent employs a partition of unity to construct asymptotics of integrals.
Linkwitz–Riley filter A Linkwitz–Riley (L-R) filter is an infinite impulse response filter used in Linkwitz–Riley audio crossovers, named after its inventors Siegfried Linkwitz and Russ Riley. This filter type was originally described in ''Active Crossover Ne ...
is an example of practical implementation of partition of unity to separate input signal into two output signals containing only high- or low-frequency components. The Bernstein polynomials of a fixed degree ''m'' are a family of ''m''+1 linearly independent polynomials that are a partition of unity for the unit interval ,1/math>. Partitions of unity are used to establish global smooth approximations for Sobolev functions in bounded domains.


See also

* * Gluing axiom * Fine sheaf


References

* , see chapter 13


External links


General information on partition of unity
at athworldbr>Applications of a partition of unity
at
lanet Math Lanet is a commune in the Aude department in south-western France. Geography The commune is located in the Corbières Massif. The village lies in the middle of the commune, on the right bank of the Orbieu, which flows northwest through the c ...
{{DEFAULTSORT:Partition Of Unity Differential topology Topology