Wallpaper Symmetry
A wallpaper pattern has translational symmetry in two independent directions.
“Look! A pattern!”
Translational symmetry wallpaper
Mirror Symmetry This mirror reflection is a symmetry of the pattern Mirror axis
Rotational Symmetry
6-center3-center 2-center Can symmetries be selected independently?
Rotational Symmetry 3-center 2-center There are mathematical limitations on decorative art! By group property, if you have some symmetries, you must have all others that follow by composition.
Classify wallpaper patterns by symmetries
Too many to list! Mirrors 4-CentersGlide reflections
Group concept: Compose two symmetries, get a symmetry
Build up the group: All symmetries generated by a few
Simplify: Track symmetries in a “fundamental cell”
Group concept: a few symmetries can generate all
p4g can be generated by one mirror, one rotation
Two patterns have the same “type” if their symmetry groups are the same
Same or different?
Each has 3-centers and mirrors
Every subgroup that fixes a point has mirrors and rotations Some subgroup that fixes a point has no mirrors p3m1p31m
“symmetric yet organic”
Wallpaper surprise: Exactly 17 types
Wallpaper surprise: Exactly 17 types Can’t have 6-fold rotation without 3- and 2-fold as well
Wallpaper surprise: Exactly 17 isomorphism classes Proved by Federov 1891 Proved by Federov 1891 Polya’s 1924 version (with figures) known to M.C. Escher Polya’s 1924 version (with figures) known to M.C. Escher See “The Polya-Escher Connection” by Doris Schattschneider in Mathematics Magazine See “The Polya-Escher Connection” by Doris Schattschneider in Mathematics Magazine
M.C. Escher’s Angel-Devil (No. 45)
Chalk Slam at Carleton
See them everywhere! pmg pattern from The Shining
Lightening Round
Tomorrow: color-reversing symmetry