Quadratically closed field

In mathematics, a quadratically closed field is a field in which every element has a square root.[1][2]

Examples
  • The field of complex numbers is quadratically closed; more generally, any algebraically closed field is quadratically closed.
  • The field of real numbers is not quadratically closed as it does not contain a square root of -1.
  • The union of the finite fields for n ≥ 0 is quadratically closed but not algebraically closed.[3]
  • The field of constructible numbers is quadratically closed but not algebraically closed.[4]

Properties
  • A field is quadratically closed if and only if it has universal invariant equal to 1.
  • Every quadratically closed field is a Pythagorean field but not conversely (for example, R is Pythagorean); however, every non-formally real Pythagorean field is quadratically closed.[2]
  • A field is quadratically closed if and only if its Witt–Grothendieck ring is isomorphic to Z under the dimension mapping.[3]
  • A formally real Euclidean field E is not quadratically closed (as -1 is not a square in E) but the quadratic extension E(√−1) is quadratically closed.[4]
  • Let E/F be a finite extension where E is quadratically closed. Either -1 is a square in F and F is quadratically closed, or -1 is not a square in F and F is Euclidean. This "going-down theorem" may be deduced from the Diller–Dress theorem.[5]

Quadratic closure

A quadratic closure of a field F is a quadratically closed field containing F which embeds in any quadratically closed field containing F. A quadratic closure for any given F may be constructed as a subfield of the algebraic closure Falg of F, as the union of all iterated quadratic extensions of F in Falg.[4]

Examples
  • The quadratic closure of R is C.[4]
  • The quadratic closure of F5 is the union of the .[4]
  • The quadratic closure of Q is the field of constructible numbers.
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References
  1. Lam (2005) p. 33
  2. Rajwade (1993) p. 230
  3. Lam (2005) p. 34
  4. Lam (2005) p. 220
  5. Lam (2005) p.270
  • 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.
  • Rajwade, A. R. (1993). Squares. London Mathematical Society Lecture Note Series. 171. Cambridge University Press. ISBN 0-521-42668-5. Zbl 0785.11022.
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