Derived set (mathematics)
In mathematics, more specifically in point-set topology, the derived set of a subset of a topological space is the set of all limit points of It is usually denoted by
The concept was first introduced by Georg Cantor in 1872 and he developed set theory in large part to study derived sets on the real line.
Definition
[edit]The derived set of a subset of a topological space denoted by is the set of all points that are limit points of that is, points such that every neighbourhood of contains a point of other than itself.
Examples
[edit]If is endowed with its usual Euclidean topology then the derived set of the half-open interval is the closed interval
Consider with the topology (open sets) consisting of the empty set and any subset of that contains 1. The derived set of is [1]
Properties
[edit]Let denote a topological space in what follows.
If and are subsets of the derived set has the following properties:[2]
- implies
- implies
A set is closed precisely when [1] that is, when contains all its limit points. For any the set is closed and is the closure of (that is, the set ).[3]
Closedness of derived sets
[edit]The derived set of a set need not be closed in general. For example, if with the indiscrete topology, the set has derived set which is not closed in But the derived set of a closed set is always closed.[proof 1]
For a point the derived set of the singleton is the set consisting of the points in the closure of and different from A space is called a TD space[4] if the derived set of every singleton in is closed; that is, if is closed for every in other words, if every point is isolated in A space has the property that is closed for all sets if and only if it is a TD space.[5]
Every TD space is a T0 space.[6]
Every T1 space is a TD space,[6] since every singleton is closed, hence which is closed. Consequently, in a T1 space, the derived set of any set is closed.[7][8]
The relation between these properties can be summarized as
The implications are not reversible. For example, the Sierpiński space is TD and not T1. And the right order topology on is T0 and not TD.
More properties
[edit]Two subsets and are separated precisely when they are disjoint and each is disjoint from the other's derived set [9]
A bijection between two topological spaces is a homeomorphism if and only if the derived set of the image (in the second space) of any subset of the first space is the image of the derived set of that subset.[10]
In a T1 space, the derived set of any finite set is empty and furthermore, for any subset and any point of the space. In other words, the derived set is not changed by adding to or removing from the given set a finite number of points.[11]
A set with (that is, contains no isolated points) is called dense-in-itself. A set with is called a perfect set.[12] Equivalently, a perfect set is a closed dense-in-itself set, or, put another way, a closed set with no isolated points. Perfect sets are particularly important in applications of the Baire category theorem.
The Cantor–Bendixson theorem states that any Polish space can be written as the union of a countable set and a perfect set. Because any Gδ subset of a Polish space is again a Polish space, the theorem also shows that any Gδ subset of a Polish space is the union of a countable set and a set that is perfect with respect to the induced topology.
Topology in terms of derived sets
[edit]Because homeomorphisms can be described entirely in terms of derived sets, derived sets have been used as the primitive notion in topology. A set of points can be equipped with an operator mapping subsets of to subsets of such that for any set and any point :
- implies
- implies
Calling a set closed if will define a topology on the space in which is the derived set operator, that is,
Cantor–Bendixson rank
[edit]For ordinal numbers the -th Cantor–Bendixson derivative of a topological space is defined by repeatedly applying the derived set operation using transfinite recursion as follows:
- for limit ordinals
The transfinite sequence of Cantor–Bendixson derivatives of is decreasing and must eventually be constant. The smallest ordinal such that is called the Cantor–Bendixson rank of
This investigation into the derivation process was one of the motivations for introducing ordinal numbers by Georg Cantor.
See also
[edit]- Adherent point – Point that belongs to the closure of some given subset of a topological space
- Condensation point – a stronger analog of limit point
- Isolated point – Point of a subset S around which there are no other points of S
- Limit point – Cluster point in a topological space
Notes
[edit]- ^ a b Baker 1991, p. 41
- ^ Pervin 1964, p.38
- ^ Baker 1991, p. 42
- ^ Aull, C. E.; Thron, W. J. (1962). "Separation axioms between T0 and T1" (PDF). Nederl. Akad. Wetensch. Proc. Ser. A. 65: 26–37. Zbl 0108.35402.Definition 3.1
- ^ Aull & Thron 1962, Theorem 5.1.
- ^ a b Goubault-Larrecq, Jean. "TD spaces". Non-Hausdorff Topology and Domain Theory.
- ^ Engelking 1989, p. 47
- ^ "Proving the derived set E' is closed".
- ^ Pervin 1964, p. 51
- ^ Hocking, John G.; Young, Gail S. (1988) [1961], Topology, Dover, p. 4, ISBN 0-486-65676-4
- ^ Kuratowski 1966, p.77
- ^ Pervin 1964, p. 62
Proofs
- ^ Proof: Assuming is a closed subset of which shows that take the derived set on both sides to get that is, is closed in
References
[edit]- Baker, Crump W. (1991), Introduction to Topology, Wm C. Brown Publishers, ISBN 0-697-05972-3
- Engelking, Ryszard (1989). General Topology. Heldermann Verlag, Berlin. ISBN 3-88538-006-4.
- Kuratowski, K. (1966), Topology, vol. 1, Academic Press, ISBN 0-12-429201-1
- Pervin, William J. (1964), Foundations of General Topology, Academic Press
Further reading
[edit]- Kechris, Alexander S. (1995). Classical Descriptive Set Theory (Graduate Texts in Mathematics 156 ed.). Springer. ISBN 978-0-387-94374-9.
- Sierpiński, Wacław F.; translated by Krieger, C. Cecilia (1952). General Topology. University of Toronto Press.