Interactive pages for Algebraic Topology

Various surfaces	Section 1
A cube with holes
Trefoil knot	Section 1
Folding a square to make a torus
The torus as a quotient of the plane
Letters of the alphabet	Section 1
Letters grouped by type	Section 1
Cage with two or three holes	Section 1
Simplices	Section 1
Barycentric coordinates
Different triangulations of the sphere	Section 1
The triangle and the square	Section 1
Skeleta of simplices	Section 1
Open sets	Definition 3.7
Finite element model
A cylinder is homeomorphic to an annulus	Example 4.9
Stereographic projection	Example 4.13
$\mathbb{R}P^1$ is homeomorphic to $S^1$	Example 7.23

Gluing two discs to make a sphere	Example 7.24
Loops on the sphere
Loops on the torus
A homotopy of the trefoil
Geometry of the Möbius strip	Example 9.21
The Möbius strip and the circle	Example 9.21
The punctured plane	Example 9.21
The punctured sphere
The punctured torus	Proposition 20.5
Wrapping an annulus
The exponential map
The exponential map is a covering	Proposition 22.7
Path lifting	Proposition 22.10
Homotopy lifting
Subdivision of a prism $[0,1]\times\Delta_2$	Section 18
Boundary of a tetrahedron	Example 10.13
Barycentric subdivision of a tetrahedron	Section 18
The map $\mu\colon|K'|\to|K|$