Quadratically closed field

• 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 ${\displaystyle \mathbb {F} _{5^{2^{n}}}}$ for n ≥ 0 is quadratically closed but not algebraically closed.^[3]

• 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]

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 F^alg of F, as the union of all iterated quadratic extensions of F in F^alg.^[4]

• The quadratic closure of R is C.^[4]
• The quadratic closure of ${\displaystyle \mathbb {F} _{5}}$ is the union of the ${\displaystyle \mathbb {F} _{5^{2^{n}}}}$.^[4]
• The quadratic closure of Q is the field of complex constructible numbers.