Quasi-algebraically closed field

In mathematics, a field F is called quasi-algebraically closed (or C₁) if every non-constant homogeneous polynomial P over F has a non-trivial zero provided the number of its variables is more than its degree. The idea of quasi-algebraically closed fields was investigated by C. C. Tsen, a student of Emmy Noether, in a 1936 paper ; and later by Serge Lang in his 1951 Princeton University dissertation and in his 1952 paper . The idea itself is attributed to Lang's advisor Emil Artin.

Formally, if P is a non-constant homogeneous polynomial in variables

X₁, ..., X_N,

and of degree d satisfying

d < N

then it has a non-trivial zero over F; that is, for some x_i in F, not all 0, we have

P(x₁, ..., x_N) = 0.

In geometric language, the hypersurface defined by P, in projective space of degree N − 2, then has a point over F.

Examples

• Any algebraically closed field is quasi-algebraically closed. In fact, any homogeneous polynomial in at least two variables over an algebraically closed field has a non-trivial zero.
• Any finite field is quasi-algebraically closed by the Chevalley–Warning theorem.
• Algebraic function fields of dimension 1 over algebraically closed fields are quasi-algebraically closed by Tsen's theorem.
• The maximal unramified extension of a complete field with a discrete valuation and a perfect residue field is quasi-algebraically closed.
• A complete field with a discrete valuation and an algebraically closed residue field is quasi-algebraically closed by a result of Lang.
• A pseudo algebraically closed field of characteristic zero is quasi-algebraically closed.

Properties

• Any algebraic extension of a quasi-algebraically closed field is quasi-algebraically closed.
• The Brauer group of a finite extension of a quasi-algebraically closed field is trivial.
• A quasi-algebraically closed field has cohomological dimension at most 1.

C_k fields

Quasi-algebraically closed fields are also called C₁. A C_k field, more generally, is one for which any homogeneous polynomial of degree d in N variables has a non-trivial zero, provided

d^k < N,

for k ≥ 1. The condition was first introduced and studied by Lang. If a field is C_i then so is a finite extension. The C₀ fields are precisely the algebraically closed fields.

Lang and Nagata proved that if a field is C_k, then any extension of transcendence degree n is C_k+n. The smallest k such that K is a C_k field ($\mathrm{\infty }$ if no such number exists), is called the diophantine dimension dd(K) of K.

C₁ fields

Every finite field is C₁.

C₂ fields

Properties

Suppose that the field k is C₂.

• Any skew field D finite over k as centre has the property that the reduced norm D^∗ → k^∗ is surjective.
• Every quadratic form in 5 or more variables over k is isotropic.

Artin's conjecture

Artin conjectured that p-adic fields were C₂, but Guy Terjanian found p-adic counterexamples for all p. The Ax–Kochen theorem applied methods from model theory to show that Artin's conjecture was true for Q_p with p large enough (depending on d).

Weakly C_k fields

A field K is weakly C_k,d if for every homogeneous polynomial of degree d in N variables satisfying

d^k < N

the Zariski closed set V(f) of Pⁿ(K) contains a subvariety which is Zariski closed over K.

A field that is weakly C_k,d for every d is weakly C_k.

Properties

• A C_k field is weakly C_k.
• A perfect PAC weakly C_k field is C_k.
• A field K is weakly C_k,d if and only if every form satisfying the conditions has a point x defined over a field which is a primary extension of K.
• If a field is weakly C_k, then any extension of transcendence degree n is weakly C_k+n.
• Any extension of an algebraically closed field is weakly C₁.
• Any field with procyclic absolute Galois group is weakly C₁.
• Any field of positive characteristic is weakly C₂.
• If the field of rational numbers $\mathbb{\mathbb{Q}}$ and the function fields ${\mathbb{𝔽}}_p(t)$ are weakly C₁, then every field is weakly C₁.

See also

• Brauer's theorem on forms
• Tsen rank