Algebraic Groups of Type D_4, Triality, and Composition Algebras

Conjugacy classes of outer automorphisms of order 3 of simple algebraic groups of classical type $D_4$ are classified over arbitrary fields. There are two main types of conjugacy classes. For one type the fixed algebraic groups are simple of type $G_2$; for the other type they are simple of type $A_2$ when the characteristic is different from 3 and are not smooth when the characteristic is 3. A large part of the paper is dedicated to the exceptional case of characteristic 3. A key ingredient of the classification of conjugacy classes of trialitarian automorphisms is the fact that the fixed groups are automorphism groups of certain composition algebras.

2010 Mathematics Subject Classification: 20G15, 11E57, 17A75, 14L10.

Keywords and Phrases: Algebraic group of type D_4, triality, outer automorphism of order 3, composition algebra, symmetric composition, octonions, Okubo algebra.

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