Topology

Open sets, continuity, compactness, connectedness: geometry without distance.

Lessons

Introductory Metric spaces and open sets Distance made abstract; open balls, open and closed sets, continuity. distance between (1,2) and (4,6) Core Compactness and connectedness The two properties that make theorems work: extreme values and intermediate values. critical points of x^3 - 3x Advanced Homeomorphism and topological invariants When two spaces are "the same", and what numbers survive stretching. 12 - 30 + 20

What survives when you are allowed to stretch but not tear. Topology makes "nearby" precise without a ruler, and its invariants (connected pieces, holes) are the coarsest and most robust things a space has.

More lessons

Spaces

Topological space Open set Closed set Metric space Basis (topology)Neighbourhood (mathematics)Interior (topology)Closure (topology)Boundary (topology)Subspace topology Product topology Quotient space (topology)

Continuity and separation

Continuous function Homeomorphism Hausdorff space Separation axiom Regular space Normal space Urysohn's lemma Tietze extension theorem

Compactness and connectedness

Compact space Connected space Path (topology)Heine–Borel theorem Tychonoff's theorem Baire category theorem Sequential compactness Locally compact space

Where it leads

Manifold Euler characteristic Surface (topology)Klein bottle Möbius strip Torus Knot theory Fixed-point theorem

Some of these lessons are adapted from open texts: Wikipedia. Each lesson page names its source and licence.

Symbols used here

Tap any symbol for the full definition, a picture, and what every letter in it means.

Questions people ask

Why "a doughnut is a coffee cup"?

Each can be deformed into the other without cutting or gluing: one hole each. Topology studies exactly the properties such deformations preserve.

What is compactness for?

It is the property that makes "every sequence has a convergent subsequence" and "continuous functions attain their maximum" true. On the real line it means closed and bounded.

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