General topology

Cocountable topology

The cocountable topology or countable complement topology on any set X consists of the empty set and all cocountable subsets of X, that is all sets whose complement in X is countable. It follows that the only closed subsets are X and the countable subsets of X. Symbolically, one writes the topology as Every set X with the cocountable topology is Lindelöf, since every nonempty open set omits only countably many points of X. It is also T1, as all singletons are closed. If X is an uncountable set then any two nonempty open sets intersect, hence the space is not Hausdorff. However, in the cocountable topology all convergent sequences are eventually constant, so limits are unique. Since compact sets in X are finite subsets, all compact subsets are closed, another condition usually related to Hausdorff separation axiom. The cocountable topology on a countable set is the discrete topology. The cocountable topology on an uncountable set is hyperconnected, thus connected, locally connected and pseudocompact, but neither weakly countably compact nor countably metacompact, hence not compact. (Wikipedia).

Video thumbnail

Topology (What is a Topology?)

What is a Topology? Here is an introduction to one of the main areas in mathematics - Topology. #topology Some of the links below are affiliate links. As an Amazon Associate I earn from qualifying purchases. If you purchase through these links, it won't cost you any additional cash, b

From playlist Topology

Video thumbnail

Sequential Compactness

In this video, I discuss the notion of sequential compactness, which is an important concept used in topology and analogy. I also explain the similarities and differences between sequential compactness and covering compactness. Compactness: https://youtu.be/xiWizwjpt8o Bolzano-Weierstrass

From playlist Topology

Video thumbnail

What is a closed set ?

I define closed sets, an important notion in topology and analysis. It is defined in terms of limit points, and has a priori nothing to do with open sets. Yet I show the important result that a set is closed if and only if its complement is open. More topology videos can be found on my pla

From playlist Topology

Video thumbnail

Every Set with the Cofinite Topology is Compact

In this video I will show you how to prove that every set with the cofinite topology is a compact topological space. If you enjoyed this video please consider sharing, liking, or subscribing. You can also help support my channel by becoming a member https://www.youtube.com/channel/UCr7lm

From playlist Topology

Video thumbnail

Properties of Compactness

Compact sets enjoy some mysterious properties, which I'll discuss in this video. More precisely, compact sets are always bounded and closed. The beauty of this result lies in the proof, which is an elegant application of this subtle concept. Enjoy! Compactness Definition: https://youtu.be

From playlist Topology

Video thumbnail

Connectedness

In this video, I define connectedness, which is a very important concept in topology and math in general. Essentially, it means that your space only consists of one piece, whereas disconnected spaces have two or more pieces. I also define the related notion of path-connectedness. Topology

From playlist Topology

Video thumbnail

Topology 1.3 : Basis for a Topology

In this video, I define what a basis for a topology is. Email : fematikaqna@gmail.com Code : https://github.com/Fematika/Animations Notes : None yet

From playlist Topology

Video thumbnail

Algebraic Topology - 5.1 - Mappings Spaces and the Compact Open Topology

We define the compact open topology on mapping spaces.

From playlist Algebraic Topology

Video thumbnail

Compactness

The single, most important concept in topology and analysis: Compactness. This is explained via covers, which I'll define as well. There are tons of applications of this concept, which you can find in the playlist below Topology Playlist: https://youtube.com/playlist?list=PLJb1qAQIrmmA13v

From playlist Topology

Video thumbnail

What is a Manifold? Lesson 1: Point Set Topology and Topological Spaces

This will begin a short diversion into the subject of manifolds. I will review some point set topology and then discuss topological manifolds. Then I will return to the "What is a Tensor" series. It has been well over a year since we began this project. We now have a Patreon Page: https

From playlist What is a Manifold?

Video thumbnail

Topological Spaces: The Subspace Topology

Today, we discuss the subspace topology, which is a useful tool to construct new topologies.

From playlist Topology & Manifolds

Video thumbnail

Classical and Digital Topological Groups

A research talk presented at the Fairfield University Mathematics Research Seminar, October 6, 2022. Should be accessible to a general mathematics audience, combining ideas from topology, graph theory, and abstract algebra. The paper is by me and Dae Woong Lee, available here: https://arx

From playlist Research & conference talks

Video thumbnail

What is a Manifold? Lesson 4: Countability and Continuity

In this lesson we review the idea of first and second countability. Also, we study the topological definition of a continuous function and then define a homeomorphism.

From playlist What is a Manifold?

Video thumbnail

What Is Network Topology? | Types of Network Topology | BUS, RING, STAR, TREE, MESH | Simplilearn

In this video on Network Topology, we will understand What is Network topology, the role of using topology while designing a network, Different types of Topologies in a Network. Network topology provides us with a way to configure the most optimum network design according to our requiremen

From playlist Cyber Security Playlist [2023 Updated]🔥

Video thumbnail

What is a Manifold? Lesson 3: Separation

He we present some alternative topologies of a line interval and then discuss the notion of separability. Note the error at 4:05. Sorry! If you are viewing this on a mobile device, my annotations are not visible. This is due to a quirck of YouTube.

From playlist What is a Manifold?

Video thumbnail

An Introduction to Topological Spaces [Lori Ziegelmeier]

This tutorial provides an introduction to topological spaces, including a brief overview of some of the central mathematicians to define a topology. The primary focus is on the set of the real line with various notions of topologies defined on this set. This tutorial was contributed as pa

From playlist Tutorial-a-thon 2021 Spring

Video thumbnail

What is a Manifold? Lesson 6: Topological Manifolds

Topological manifolds! Finally! I had two false starts with this lesson, but now it is fine, I think.

From playlist What is a Manifold?

Video thumbnail

SEPARATION BUT MATHEMATICALLY: What Types of Mathematical Topologies are there? | Nathan Dalaklis

The title of this video is a bit convoluted. What do you mean by "Separation but Mathematically"? Well, in this video I'll be giving a (very diluted) answer to the question "What types of mathematical topologies are there?" by introducing the separation axioms in topology. The separation

From playlist The New CHALKboard

Video thumbnail

Algebraic Topology - 5.3 - Mapping Spaces and the Compact Open Topology

Description of the adjunction (X \times -, Top(X,-))

From playlist Algebraic Topology

Video thumbnail

Topological Spaces: The Standard Topology on R^n

Today, we construct the standard topology, which gives us the way we usually think about R^n.

From playlist Topology & Manifolds

Related pages

Metacompact space | Compact space | Locally connected space | List of topologies | Counterexamples in Topology | Complement (set theory) | Countable set | Connected space | Limit point compact | Hyperconnected space | Lindelöf space | Pseudocompact space | Empty set | T1 space | Hausdorff space | Open set