Research output: Chapter in Book/Report/Conference proceeding › Conference contribution

Title of host publication | Symmetries in Graphs, Maps, and Polytopes |
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Date | Accepted/In press - 2015 |

Date | Published (current) - Apr 2016 |

Pages | 81-95 |

Publisher | Springer-Verlag |

Original language | English |

ISBN (Electronic) | 978-3-319-30451-9 |

ISBN (Print) | 978-3-319-30449-6 |

Name | Springer Proceedings in Mathematics & Statistics |
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Publisher | Springer |

Volume | 159 |

ISSN (Print) | 2194-1009 |

In this paper, we discuss a Clifford algebra framework for discrete symmetries -- e.g. reflection, Coxeter, conformal, modular groups -- that also leads to a surprising number of new results in itself. Clifford algebra affords a particularly simple description for performing reflections (via `sandwiching' with vectors in the Clifford algebra), and since via the Cartan-Dieudonn\'e theorem all orthogonal transformations can be written as products of reflections, all such operations can be performed via `sandwiching' with Clifford algebra multivectors. We begin by viewing the largest non-crystallographic reflection/Coxeter group $H_4$ as a group of rotations in two different ways -- firstly via a folding from the largest exceptional group $E_8$, and secondly by induction from the icosahedral group $H_3$ via Clifford spinors. We then generalise this latter observation and present a procedure by which starting with any 3D root system one constructs a corresponding 4D root system. This affords a new -- spinorial -- perspective on 4D phenomena, in particular as the induced root systems are precisely the exceptional ones in 4D, and their unusual automorphism groups are easily explained in the spinorial picture; we discuss the wider context of Platonic solids, Arnold's trinities and the McKay correspondence. The multivector groups can be used to perform concrete group theoretic calculations, e.g. those for $H_3$ and $E_8$, and we discuss how various representations can also be constructed in this Clifford framework; in particular, representations of quaternionic type arise very naturally.

Dechant, P.P., 2014. A 3D spinorial view of 4D exceptional phenomena. In Symmetries in Graphs, Maps, and Polytopes (pp. 81-95). Springer International Publishing.

## Yau Institute: Tsinghua Summer Workshop in Geometry and Physics 2017

Activity: Talk or presentation › Workshop

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