This old-school website, put together by Japanese electronic engineer Tadao Ito, explains non-Euclidean and hyperbolic geometry, projective geometry, and some properties of the figure-eight shape (genus 2 torus) – with some lovely diagrams. Worth a dig through! This is part of the Aperiodical Advent Calendar. We’ll be posting a new surprise for you each morning until…

Here’s a nice set of origami tutorials on how to make nice Christmas decorations from coloured paper, using Sonobe modules. There’s instructions for a cube (nice for putting small gifts inside), stellated octahedron and stellated icosahedron, both of which look like pretty stars. Oooh. This is part of the Aperiodical Advent Calendar. We’ll be posting…

Today’s entry is a Theorem of the Day: The Euclid-Euler Theorem: An even positive integer is a perfect number, that is, equals the sum of its proper divisors, if and only if it has the form $2^{n−1}(2^n − 1)$, for some n such that $2^n − 1$ is prime. This theorem describes the relationship between…

This is part of the Aperiodical Advent Calendar. We’ll be posting a new surprise for you each morning until Christmas!