VII - The two Tits constructions

Summary

This chapter is concerned with the two Tits constructions of cubic Jordan algebras over commutative rings, which we present for the first time in book form. The key feature of the first Tits construction is that it starts out from cubic alternative algebras rather than cubic associative ones; the key notion that keeps the construction going is that of a Kummer element. Similar statements apply to the second Tits construction, where Kummer elements are replaced by étale elements. Such elements are available in abundance over residually big LG rings. It follows that all Albert algebras over such rings or over arbitrary fields may be obtained from the second Tits construction. The chapter concludes with an application to cubic Jordan division algebras over fields. We show that they are either purely inseparable field extensions of characteristic 3 or Freudenthal algebras of dimension 1, 3, 9, or 27. In each of these dimensions, we construct examples over appropriate fields and conclude the section by showing that over the “standard” fields (the complex numbers, the real numbers, finite, local and global ones), Albert division algebras do not exist.