Pfister form

In mathematics, a Pfister form is a particular kind of quadratic form, introduced by Albrecht Pfister in 1965. In what follows, quadratic forms are considered over a field F of characteristic not 2. For a natural number n, an n-fold Pfister form over F is a quadratic form of dimension 2ⁿ that can be written as a tensor product of quadratic forms

$\langle\!\langle a_1, a_2, ... , a_n \rangle\!\rangle \cong \langle 1, -a_1 \rangle \otimes \langle 1, -a_2 \rangle \otimes ... \otimes \langle 1, -a_n \rangle,$

for some nonzero elements a₁, ..., a_n of F.^[1] (Some authors omit the signs in this definition; the notation here simplifies the relation to Milnor K-theory, discussed below.) An n-fold Pfister form can also be constructed inductively from an (n–1)-fold Pfister form q and a nonzero element a of F, as $q \oplus (-a)q$.

So the 1-fold and 2-fold Pfister forms look like:

$\langle\!\langle a\rangle\!\rangle\cong \langle 1, -a \rangle = x^2 - ay^2$.

$\langle\!\langle a,b\rangle\!\rangle\cong \langle 1, -a, -b, ab \rangle = x^2 - ay^2 - bz^2 + abw^2.$

For n ≤ 3, the n-fold Pfister forms are norm forms of composition algebras.^[2] In that case, two n-fold Pfister forms are isomorphic if and only if the corresponding composition algebras are isomorphic.

The n-fold Pfister forms additively generate the n-th power Iⁿ of the fundamental ideal of the Witt ring of F.^[2]

Characterizations

A quadratic form q over a field F is multiplicative if, for vectors of indeterminates x and y, we can write q(x).q(y) = q(z) for some vector z of rational functions in the x and y over F. Isotropic quadratic forms are multiplicative.^[3] For anisotropic quadratic forms, Pfister forms are multiplicative, and conversely.^[4]

For n-fold Pfister forms with n ≤ 3, this had been known since the 19th century; in that case z can be taken to be bilinear in x and y, by the properties of composition algebras. It was a remarkable discovery by Pfister that n-fold Pfister forms for all n are multiplicative in the more general sense here, involving rational functions. For example, he deduced that for any field F and any natural number n, the set of sums of 2ⁿ squares in F is closed under multiplication, using that the quadratic form $x_1^2 +\cdots + x_{2^n}^2$ is an n-fold Pfister form (namely, $\langle\!\langle -1, ... , -1 \rangle\!\rangle$).^[5]

Another striking feature of Pfister forms is that every isotropic Pfister form is in fact hyperbolic, that is, isomorphic to a direct sum of copies of the hyperbolic plane $\langle 1, -1 \rangle$. This property also characterizes Pfister forms, as follows. If q is an anisotropic quadratic form over a field F, and if q becomes hyperbolic over every extension field E such that q becomes isotropic over E, then q is isomorphic to aφ for some nonzero a in F and some Pfister form φ over F.^[6]

Connection with K-theory

Let k_n(F) be the n-th Milnor K-group modulo 2. There is a homomorphism from k_n(F) to the quotient Iⁿ/Iⁿ⁺¹ in the Witt ring of F, given by

$\{a_1,\ldots,a_n\} \mapsto \langle\!\langle a_1, a_2, ... , a_n \rangle\!\rangle ,$

where the image is an n-fold Pfister form.^[7] The homomorphism is surjective, since the Pfister forms additively generate Iⁿ. One part of the Milnor conjecture, proved by Orlov, Vishik and Voevodsky, states that this homomorphism is in fact an isomorphism k_n(F) ≅ Iⁿ/Iⁿ⁺¹.^[8] That gives an explicit description of the abelian group Iⁿ/Iⁿ⁺¹ by generators and relations. The other part of the Milnor conjecture, proved by Voevodsky, says that k_n(F) (and hence Iⁿ/Iⁿ⁺¹) maps isomorphically to the Galois cohomology group Hⁿ(F, F₂).

Pfister neighbors

A Pfister neighbor is an anisotropic form σ which is isomorphic to a subform of aφ for some nonzero a in F and some Pfister form φ with dim φ < 2 dim σ.^[9] The associated Pfister form φ is determined up to isomorphism by σ. Every anisotropic form of dimension 3 is a Pfister neighbor; an anisotropic form of dimension 4 is a Pfister neighbor if and only if its discriminant in F^*/(F^*)² is trivial.^[10] A field F has the property that every 5-dimensional anisotropic form over F is a Pfister neighbor if and only if it is a linked field.^[11]

Notes

1. Elman, Karpenko, Merkurjev (2008), section 9.B.
2. Lam (2005) p.316
3. Lam (2005) p.324
4. Lam (2005) p.325
5. Lam (2005) p.319
6. Elman, Karpenko, Merkurjev (2008), Corollary 23.4.
7. Elman, Karpenko, Merkurjev (2008), section 5.
8. Orlov, Vishik, Voevodsky (2007).
9. Elman, Karpenko, Merkurjev (2008), Definition 23.10.
10. Lam (2005) p.341
11. Lam (2005) p.342

References

• Elman, Richard; Karpenko, Nikita; Merkurjev, Alexander (2008), Algebraic and geometric theory of quadratic forms, American Mathematical Society, ISBN 978-0-8218-4329-1, MR 2427530
• 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, Ch. 10
• Orlov, Dmitri; Vishik, Alexander; Voevodsky, Vladimir (2007), "An exact sequence for K_*^M/2 with applications to quadratic forms", Annals of Mathematics 165: 1–13, doi:10.4007/annals.2007.165.1, MR 2276765