Albert algebra

In mathematics, an Albert algebra is a 27-dimensional exceptional Jordan algebra. They are named after Abraham Adrian Albert, who pioneered the study of non-associative algebras, usually working over the real numbers. Over the real numbers, there are three such Jordan algebras up to isomorphism. One of them, which was first mentioned by Template:Harvs and studied by Template:Harvtxt, is the set of 3×3 self-adjoint matrices over the octonions, equipped with the binary operation

$x\circ y=\frac{1}{2}(x\cdot y+y\cdot x),$

where $\cdot$ denotes matrix multiplication. Another is defined the same way, but using split octonions instead of octonions. The final is constructed from the non-split octonions using a different standard involution.

Over any algebraically closed field, there is just one Albert algebra, and its automorphism group G is the simple split group of type F₄. (For example, the complexifications of the three Albert algebras over the real numbers are isomorphic Albert algebras over the complex numbers.) Because of this, for a general field F, the Albert algebras are classified by the Galois cohomology group H¹(F,G).

The Kantor–Koecher–Tits construction applied to an Albert algebra gives a form of the E7 Lie algebra. The split Albert algebra is used in a construction of a 56-dimensional structurable algebra whose automorphism group has identity component the simply-connected algebraic group of type E₆.

The space of cohomological invariants of Albert algebras a field F (of characteristic not 2) with coefficients in Z/2Z is a free module over the cohomology ring of F with a basis 1, f₃, f₅, of degrees 0, 3, 5. The cohomological invariants with 3-torsion coefficients have a basis 1, g₃ of degrees 0, 3. The invariants f₃ and g₃ are the primary components of the Rost invariant.

See also

• Euclidean Jordan algebra for the Jordan algebras considered by Jordan, von Neumann and Wigner
• Euclidean Hurwitz algebra for details of the construction of the Albert algebra for the octonions

References

• Template:Citation
• Template:Citation
• Template:Cite book
• Template:Cite book
• Template:Citation
• Template:Citation
• Template:Citation
• Template:Citation

Further reading

• Albert algebra at Encyclopedia of Mathematics.