Linked quaternion algebras
Let F be a field of characteristic not equal to 2. Let A = (a1,a2) and B = (b1,b2) be quaternion algebras over F. The algebras A and B are linked quaternion algebras over F if there is x in F such that A is equivalent to (x,y) and B is equivalent to (x,z).
The Albert form for A, B is
It can be regarded as the difference in the Witt ring of the ternary forms attached to the imaginary subspaces of A and B. The quaternion algebras are linked if and only if the Albert form is isotropic.
The following properties of F are equivalent:
- F is linked.
- Any two quaternion algebras over F are linked.
- Every Albert form (dimension six form of discriminant −1) is isotropic.
- The quaternion algebras form a subgroup of the Brauer group of F.
- Every dimension five form over F is a Pfister neighbour.
- No biquaternion algebra over F is a division algebra.
- Lam, Tsit-Yuen (2005). Introduction to Quadratic Forms over Fields. Graduate Studies in Mathematics 67. American Mathematical Society. ISBN 0-8218-1095-2. MR 2104929. Zbl 1068.11023.