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In topology and related areas of mathematics, a subset A of a topological space X is said to be dense in X if every point of X either belongs to A or else is arbitrarily "close" to a member of A — for instance, the rational numbers are a dense subset of the real numbers because every real number either is a rational number or has a rational number arbitrarily close to it (see Diophantine approximation).
Formally, is dense in if the smallest closed subset of containing is itself.[1]
The density of a topological space is the least cardinality of a dense subset of
^Steen, L. A.; Seebach, J. A. (1995), Counterexamples in Topology, Dover, ISBN 0-486-68735-X
showing that densesets need not contain any non-empty open set. The intersection of two dense open subsets of a topological space is again dense and open...
topological space is called nowhere dense or rare if its closure has empty interior. In a very loose sense, it is a set whose elements are not tightly clustered...
X} will be a topological space. The definition of meagre set uses the notion of a nowhere dense subset of X , {\displaystyle X,} that is, a subset of X...
Every dense Gδ set in a Baire space is a Baire space. The result need not hold if the Gδ set is not dense. See the Examples section. Every comeagre set in...
the Mandelbrot set. For example, Shishikura proved that, for a denseset of parameters in the boundary of the Mandelbrot set, the Julia set has Hausdorff...
{\displaystyle \operatorname {J} (f)} is a nowhere denseset (it is without interior points) and an uncountable set (of the same cardinality as the real numbers)...
discrete sets, relatively densesets, and Delone sets (named after Boris Delone) are several closely related definitions of well-spaced sets of points...
In mathematics, a partial order or total order < on a set X {\displaystyle X} is said to be dense if, for all x {\displaystyle x} and y {\displaystyle...
countable collection of open densesets is dense basic set theory 1. Naive set theory 2. A weak set theory, given by Kripke–Platek set theory without the axiom...
however, do not necessarily consist of isolated points and may even be dense in an interval (a, b). For example, for any summable sequence ( a i ) (a_{i})...
it has a discontinuity there. The set of all points of discontinuity of a function may be a discrete set, a denseset, or even the entire domain of the...
intersection of any countable collection of dense open sets is dense; see Baire space. Baire space is the set of all functions from the natural numbers...
family of maps from the complex plane to itself is hyperbolic for an open denseset of parameters. Furstenberg conjecture – is every invariant and ergodic...
is an alternative to polygonal modeling. An object is represented by a denseset of points or viewer-facing discs holding lighting information. Surfels...
In mathematics, a Meyer set or almost lattice is a relatively denseset X of points in the Euclidean plane or a higher-dimensional Euclidean space such...
iterations xk stay inside the attractor and, with probability 1, form a denseset in the latter. The "chaos game" method plots points in random order all...
ternary construction only in passing, as an example of a perfect set that is nowhere dense (, Anmerkungen zu §10, /p. 590). More generally, in topology,...
topological space such that the intersection of countably many dense open sets is still dense). It is used in the proof of results in many areas of analysis...
union of compact convex sets is again compact and convex. Meager, nowhere dense, and Baire A disk in a TVS is not nowhere dense if and only if its closure...
contrast, if most of the elements are non-zero, the matrix is considered dense. The number of zero-valued elements divided by the total number of elements...
{\displaystyle g(x)=G'(x)=0} for all x in the set { F ( x n ) } n ≥ 1 {\displaystyle \{F(x_{n})\}_{n\geq 1}} which is dense in the interval [ F ( − 1 ) , F ( 1...
Nowhere denseset Null set, conull set Partition regular Piecewise syndetic set Schnirelmann density Small set (combinatorics) Stationary set Syndetic set Thick...
Cofinal and coinitial set, sometimes also called dense Meet-denseset and join-denseset Linked set (upwards and downwards) Directed set (upwards and downwards)...