In general topology, a subset of a topological space is **perfect** if it is closed and has no isolated points. Equivalently: the set is perfect if , where denotes the set of all limit points of , also known as the derived set of .

In a perfect set, every point can be approximated arbitrarily well by other points from the set: given any point of and any neighborhood of the point, there is another point of that lies within the neighborhood. Furthermore, any point of the space that can be so approximated by points of belongs to .

Note that the term *perfect space* is also used, incompatibly, to refer to other properties of a topological space, such as being a G_{δ} space.

Examples of perfect subsets of the real line are: the empty set, all closed intervals, the real line itself, and the Cantor set. The latter is noteworthy in that it is totally disconnected.

Every topological space can be written in a unique way as the disjoint union of a perfect set and a scattered set.^{ [1] }^{ [2] }

Cantor proved that every closed subset of the real line can be uniquely written as the disjoint union of a perfect set and a countable set. This is also true more generally for all closed subsets of Polish spaces, in which case the theorem is known as the Cantor–Bendixson theorem.

Cantor also showed that every non-empty perfect subset of the real line has cardinality , the cardinality of the continuum. These results are extended in descriptive set theory as follows:

- If
*X*is a complete metric space with no isolated points, then the Cantor space 2^{ω}can be continuously embedded into*X*. Thus*X*has cardinality at least . If*X*is a separable, complete metric space with no isolated points, the cardinality of*X*is exactly . - If
*X*is a locally compact Hausdorff space with no isolated points, there is an injective function (not necessarily continuous) from Cantor space to*X*, and so*X*has cardinality at least .

- ↑ Engelking, problem 1.7.10, p. 59
- ↑ https://math.stackexchange.com/questions/3856152

In mathematics, **cardinal numbers**, or **cardinals** for short, are a generalization of the natural numbers used to measure the cardinality (size) of sets. The cardinality of a finite set is a natural number: the number of elements in the set. The *transfinite* cardinal numbers, often denoted using the Hebrew symbol (aleph) followed by a subscript, describe the sizes of infinite sets.

In mathematics, the **cardinality** of a set is a measure of the "number of elements" of the set. For example, the set contains 3 elements, and therefore has a cardinality of 3. Beginning in the late 19th century, this concept was generalized to infinite sets, which allows one to distinguish between the different types of infinity, and to perform arithmetic on them. There are two approaches to cardinality: one which compares sets directly using bijections and injections, and another which uses cardinal numbers. The cardinality of a set is also called its **size**, when no confusion with other notions of size is possible.

In topology and related branches of mathematics, a **connected space** is a topological space that cannot be represented as the union of two or more disjoint non-empty open subsets. Connectedness is one of the principal topological properties that are used to distinguish topological spaces.

This is a glossary of some terms used in the branch of mathematics known as topology. Although there is no absolute distinction between different areas of topology, the focus here is on general topology. The following definitions are also fundamental to algebraic topology, differential topology and geometric topology.

In mathematics, a **Borel set** is any set in a topological space that can be formed from open sets through the operations of countable union, countable intersection, and relative complement. Borel sets are named after Émile Borel.

In topology and mathematics in general, the **boundary** of a subset *S* of a topological space *X* is the set of points which can be approached both from *S* and from the outside of *S*. More precisely, it is the set of points in the closure of *S* not belonging to the interior of *S*. An element of the boundary of *S* is called a **boundary point** of *S*. The term **boundary operation** refers to finding or taking the boundary of a set. Notations used for boundary of a set *S* include bd(*S*), fr(*S*), and ∂*S*. Some authors use the term **frontier** instead of boundary in an attempt to avoid confusion with a different definition used in algebraic topology and the theory of manifolds. Despite widespread acceptance of the meaning of the terms boundary and frontier, they have sometimes been used to refer to other sets. For example, the term frontier has been used to describe the **residue** of *S*, namely *S* \ *S*. Felix Hausdorff named the intersection of *S* with its boundary the **border** of *S*.

In mathematics, a **Lindelöf space** is a topological space in which every open cover has a countable subcover. The Lindelöf property is a weakening of the more commonly used notion of *compactness*, which requires the existence of a *finite* subcover.

In mathematics, a **Cantor space**, named for Georg Cantor, is a topological abstraction of the classical Cantor set: a topological space is a **Cantor space** if it is homeomorphic to the Cantor set. In set theory, the topological space 2^{ω} is called "the" Cantor space.

In topology and related areas of mathematics, a **topological property** or **topological invariant** is a property of a topological space which is invariant under homeomorphisms. That is, a property of spaces is a topological property if whenever a space *X* possesses that property every space homeomorphic to *X* possesses that property. Informally, a topological property is a property of the space that can be expressed using open sets.

In order theory, a partially ordered set *X* is said to satisfy the **countable chain condition**, or to be **ccc**, if every strong antichain in *X* is countable.

In the mathematical field of set theory, **Martin's axiom**, introduced by Donald A. Martin and Robert M. Solovay (1970), is a statement that is independent of the usual axioms of ZFC set theory. It is implied by the continuum hypothesis, but it is consistent with ZFC and the negation of the continuum hypothesis. Informally, it says that all cardinals less than the cardinality of the continuum, , behave roughly like . The intuition behind this can be understood by studying the proof of the Rasiowa–Sikorski lemma. It is a principle that is used to control certain forcing arguments.

In mathematics, more specifically in point-set topology, the **derived set** of a subset *S* of a topological space is the set of all limit points of *S*. It is usually denoted by *S* '.

In set theory, the **cardinality of the continuum** is the cardinality or "size" of the set of real numbers , sometimes called the continuum. It is an infinite cardinal number and is denoted by or .

In mathematics, **set-theoretic topology** is a subject that combines set theory and general topology. It focuses on topological questions that are independent of Zermelo–Fraenkel set theory (ZFC).

In general topology, a subset of a topological space is said to be **dense-in-itself** or **crowded** if has no isolated point. Equivalently, is dense-in-itself if every point of is a limit point of . Thus is dense-in-itself if and only if , where is the derived set of .

In mathematics, a **cardinal function** is a function that returns cardinal numbers.

In topology and related areas of mathematics, a subset *A* of a topological space *X* is called **dense** if every point *x* in *X* either belongs to *A* or is a limit point of *A*; that is, the closure of *A* is constituting the whole set *X*. Informally, for every point in *X*, the point is either in *A* or 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.

In set theory, an **ordinal number**, or **ordinal**, is one generalization of the concept of a natural number that is used to describe a way to arrange a collection of objects in order, one after another.

In the mathematical discipline of set theory, a **cardinal characteristic of the continuum** is an infinite cardinal number that may consistently lie strictly between , and the cardinality of the continuum, that is, the cardinality of the set of all real numbers. The latter cardinal is denoted or . A variety of such cardinal characteristics arise naturally, and much work has been done in determining what relations between them are provable, and constructing models of set theory for various consistent configurations of them.

In topology, a **scattered space** is a topological space *X* that contains no nonempty dense-in-itself subset. Equivalently, every nonempty subset *A* of *X* contains a point isolated in *A*.

- Engelking, Ryszard,
*General Topology*, Heldermann Verlag Berlin, 1989. ISBN 3-88538-006-4 - Kechris, A. S. (1995),
*Classical Descriptive Set Theory*, Berlin, New York: Springer-Verlag, ISBN 3540943749 - Levy, A. (1979),
*Basic Set Theory*, Berlin, New York: Springer-Verlag - edited by Elliott Pearl. (2007), Pearl, Elliott (ed.),
*Open problems in topology. II*, Elsevier, ISBN 978-0-444-52208-5, MR 2367385 CS1 maint: extra text: authors list (link)

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