Derived set (mathematics)
In mathematics, more specifically in
The concept was first introduced by
Definition
The derived set of a subset of a topological space denoted by is the set of all points that are
Examples
If is endowed with its usual
Consider with the
Properties
If and are subsets of the topological space then the derived set has the following properties:[2]
- implies
- implies
A subset of a topological space is closed precisely when [1] that is, when contains all its limit points. For any subset the set is closed and is the closure of (that is, the set ).[3]
The derived set of a subset of a space need not be closed in general. For example, if with the trivial topology, the set has derived set which is not closed in But the derived set of a closed set is always closed.[proof 1] In addition, if is a T1 space, the derived set of every subset of is closed in [4][5]
Two subsets and are separated precisely when they are disjoint and each is disjoint from the other's derived set [6]
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.[7]
A space is a T1 space if every subset consisting of a single point is closed.[8] In a T1 space, the derived set of a set consisting of a single element is empty (Example 2 above is not a T1 space). It follows that in T1 spaces, the derived set of any finite set is empty and furthermore,
A set with (that is, contains no isolated points) is called dense-in-itself. A set with is called a perfect set.[11] 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
Topology in terms of derived sets
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
For ordinal numbers the -th Cantor–
- for limit ordinals
The transfinite sequence of Cantor–Bendixson derivatives of is
This investigation into the derivation process was one of the motivations for introducing
See also
- 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
- ^ a b Baker 1991, p. 41
- ^ Pervin 1964, p.38
- ^ Baker 1991, p. 42
- ^ Engelking 1989, p. 47
- ^ "General topology - Proving the derived set $E'$ is closed".
- ^ Pervin 1964, p. 51
- ISBN 0-486-65676-4
- ^ Pervin 1964, p. 70
- ^ Kuratowski 1966, p.77
- ^ Kuratowski 1966, p.76
- ^ 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
- Baker, Crump W. (1991), Introduction to Topology, Wm C. Brown Publishers, ISBN 0-697-05972-3
- ISBN 3-88538-006-4.
- ISBN 0-12-429201-1
- Pervin, William J. (1964), Foundations of General Topology, Academic Press
Further reading
- ISBN 978-0-387-94374-9.
- Sierpiński, Wacław F.; translated by Krieger, C. Cecilia (1952). General Topology. University of Toronto Press.