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# SO(8)

In mathematics, SO(8) is the special orthogonal group acting on eight-dimensional Euclidean space. It could be either a real or complex simple Lie group of rank 4 and dimension 28.

Spin(8)

Like all special orthogonal groups of *n*> 2, SO(8) is not simply connected, having a fundamental group isomorphic to **Z**_{2}. The universal cover of SO(8) is the spin group **Spin(8)**.

Center

The center of SO(8) is **Z**_{2}, the diagonal matrices {±I} (as for all SO(2*n*) for 2*n* > 2), while the center of Spin(8) is **Z**_{2}×**Z**_{2} (as for all Spin(4*n*), 4*n* > 0).

Triality

Main article: Triality

Dynkin diagram of SO(8), (D4)

SO(8) is unique among the simple Lie groups in that its Dynkin diagram (shown right) (D_{4} under the Dynkin classification) possesses a three-fold symmetry. This gives rise to peculiar feature of Spin(8) known as triality. Related to this is the fact that the two spinor representations, as well as the fundamental vector representation, of Spin(8) are all eight-dimensional (for all other spin groups the spinor representation is either smaller or larger than the vector representation). The triality automorphism of Spin(8) lives in the outer automorphism group of Spin(8) which is isomorphic to the symmetric group S_{3} that permutes these three representations. The automorphism group acts on the center **Z**_{2} x **Z**_{2} (which also has automorphism group isomorphic to *S*_{3} which may also be considered as the general linear group over the finite field with two elements, *S*_{3} ≅GL(2,2)). When one quotients Spin(8) by one central **Z**_{2}, breaking this symmetry and obtaining SO(8), the remaining outer automorphism group is only **Z**_{2}. The triality symmetry acts again on the further quotient SO(8)/**Z**_{2}.

Sometimes Spin(8) appears naturally in an "enlarged" form, as the automorphism group of Spin(8), which breaks up as a semidirect product: Aut(Spin(8)) ≅ Spin (8) ⋊ S3.

Root system

\( (\pm 1,\pm 1,0,0) \)

\( (\pm 1,0,\pm 1,0) \)

\( (\pm 1,0,0,\pm 1) \)

\( (0,\pm 1,\pm 1,0) \)

\( (0,\pm 1,0,\pm 1) \)

\( (0,0,\pm 1,\pm 1) \)

Weyl group

Its Weyl/Coxeter group has 4!×8=192 elements.

Cartan matrix

\( \begin{pmatrix} 2 & -1 & -1 & -1\\ -1 & 2 & 0 & 0\\ -1 & 0 & 2 & 0\\ -1 & 0 & 0 & 2 \end{pmatrix} \)

See also

Octonions

Clifford algebra

G2

References

Adams, J.F. (1996), Lectures on exceptional Lie groups, Chicago Lectures in Mathematics, University of Chicago Press, ISBN 0-226-00526-7

Chevalley, Claude (1997), The algebraic theory of spinors and Clifford algebras, Collected works, 2, Springer-Verlag, ISBN 3-540-57063-2 (originally published in 1954 by Columbia University Press)

Porteous, Ian R. (1995), Clifford algebras and the classical groups, Cambridge Studies in Advanced Mathematics, 50, Cambridge University Press, ISBN 0-521-55177-3

Undergraduate Texts in Mathematics

Graduate Studies in Mathematics

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