# Topological Terms

Hey there,Here’s discussing some basic topological terms such as the most useful definitions like T0 spaces, open sets, ε-net… that are the backbone of the subject topology.

#### Topological Spaces:

A topological space (X,ℑ) is a set, X of points &a family of subsets of X which satisfy the following property:

• The union of any number of members of ℑ is a member of ℑ.
• The intersection of any finite number of members of ℑ is a member of ℑ .
• Φ & X∈(X,ℑ ); Where members of ℑ are open sets.

#### Indescrete Topology:

Topology containing only Φ & X; where Φ is null set.&X is the whole set.

#### Descrete Topology:

Topology consisting of other elements along withΦ & X; where Φ is null set.&X is the whole set.

• Indescrete topology is the smallest topology. i.e. Weakest topology.
• Descrete topology is the strongest topology. i.e. Coarcest topology.
• Union & intersection of any no. of topology is a topology.

#### Limit or Accumulation point:

x is a limit point of a subset E if & only if every open set containing x contains a point of E different from x i.e.if x∈G∈ℑ, then E∩G\{x}≠Φ.

#### Interior point:

Specifically, in topology,A point that is in the interior of the set A is the interior point.

#### Derived set in a topology:

In topology, the set of all limit points is the derived set.

Some tips for Derived sets:

• d(Φ) = Φ
• If A ⊆ B then d(A) ⊆ d(B)
• If x∈d(E), then x∈d(E\{x})
• d(A∪B)=d(A)∪d(B)

#### Open set:

In topology,A set containing all its interior point, is the open set.

#### Closed Set:

In topology,A set containing all its limit points, is the closed set.

## Topological Spaces:

#### T0 Space:

We can define a topological space T0 Space as:
If x &y are two distinct points of the set X, then there exist an open set which contain one of them but not the other.

#### T1 Space:

In topology, the T1 Space is defined as:                                                                    If x &y are two distinct points of the set X, then there exist two open sets one containing x but not y and other containing y but not x.

#### T2 Space (Hausdorff Space):

In topology,T2 Space is defined as:                                                                          If x &y are two distinct points of the set X, then there exist two open sets one containing x and other containing y.                                                                           Another name of T2 Space the Hausdorff space.

### Important notes about topological spaces:

• T2 Space is always a T1 Space & T0 Space,i.e. it always satisfy axioms of T0 Space.
• T1 Space is always a T0 Space. i.e. it always satisfy axioms of T0 Space.
• But T0 Space can never be T1 Space & T2 Space.

#### First axiom space:

A topological space X is of first axiom space, if it satisfies the following axiom of countability:

For every point x∈X; There exist a countable family {Bn(x)} of open sets containing x such that whenever x ∈ open set G.
Bn(x) ∈ G for some n.

#### Second axiom space:

A topological space X is of second axiom space if and only if it satisfies the following axiom of countability:

There exist a countable base for topology τ.

#### ε-net:

Let(X,d) be a metric space & ε > 0. A finite subset E of X is said to be ε-net for X if and only if E is finite & for every x ∈ X there exist a point e E s.t. d(e,x)<ε.
To illustrate the above definition we make use of the diagram, Here,square denotes space(X,d), The circle in figure denotes the subset of X, e is the point in E.

#### Lindelof Space:

Lindelof space is a topological space in which every open cover has a countable subcover.

#### Euclidean n-space:

Let X=Rn denotes the set of all ordered n-tuples of real numbers for fixed n=N.
if x,y ∈ Rn , then
x=<x1,x2,—xn> , y=<y1,y2,—yn>
Define a mapping
d:RnxRn→R
d[En(x,y)] = dEn(<x1,x2,—xn><y1,y2,—yn>)
= √∑i=1(xi-yi)2
The set (Rn,dEn) is the real Euclidean space.

#### Hilbert space:

Let F denotes the set of all infinite sequences i.e.if x F then, x=<x1,x2,—xn> of real numbers for which ∑i=1 x2 converges.
Let x,y ∈ H then,
x=<x1,x2,—xn>
y=<y1,y2,—yn>
Then, d11(x,y)=dh(<x1,x2,—xn><y1,y2,—yn>)
= √∑i=1(xi-yi)2
Then the space (H,dh) is metric space called Hilbert space.

#### Principle of Transfinite induction:

If X is the well ordered set & E is the subset of X with the property that Xx ⊆ E implies that x ∈ E, then E=X.

#### Axiom of choice:

Cartesion product of non empty family of non empty family of non empty of non empty sets is non empty.

There is no set containing all the ordinal numbers.

#### Finite intersection property:

We define a finite intersection property as -every finite subfamily of the family has a non empty intersection.

Here, possibly all major concepts of topology are discussed.Hopefully, these would be useful yo the reader.