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 3.8
Finite element model	Remark 2.3
A cylinder is homeomorphic to an annulus	Example 4.7
Stereographic projection	Example 4.10
$\mathbb{R}P^1$ is homeomorphic to $S^1$	Example 8.23

Gluing two discs to make a sphere	Example 8.24
Loops on the sphere
Loops on the torus
A homotopy of the trefoil
Geometry of the Möbius strip	Example 10.23
The Möbius strip and the circle	Example 10.23
The punctured plane	Example 10.23
The punctured sphere
The punctured torus	Remark 15.26
Wrapping an annulus	Example 15.27
The exponential map
The exponential map is a covering	Proposition 12.6
Path lifting	Proposition 12.9
Homotopy lifting	Proposition 12.12
Subdivision of a prism $[0,1]\times\Delta_2$
Boundary of a tetrahedron	Example 18.7
Barycentric subdivision of a tetrahedron	Example 22.5
The map $\mu\colon|K'|\to|K|$	Example 22.8