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 $\mu\colon|K'|\to|K|$