Quadratic field
In algebraic number theory, a quadratic field is an algebraic number field K of degree two over Q, the rational numbers. The map d ↦ Q(√d) is a bijection from the set of all square-free integers d ≠ 0, 1 to the set of all quadratic fields. If d > 0 the corresponding quadratic field is called a real quadratic field, and for d < 0 an imaginary quadratic field or complex quadratic field, corresponding to whether it is or not a subfield of the field of the real numbers.
Quadratic fields have been studied in great depth, initially as part of the theory of binary quadratic forms. There remain some unsolved problems. The class number problem is particularly important.
For a nonzero square free integer d, the discriminant of the quadratic field K=Q(√d) is d if d is congruent to 1 modulo 4, and otherwise 4d. For example, when d is −1 so that K is the field of so-called Gaussian rationals, the discriminant is −4. The reason for this distinction relates to general algebraic number theory. The ring of integers of K is spanned over the rational integers by 1 and √d only in the second case, while in the first case it is spanned by 1 and (1 + √d)/2.
The set of discriminants of quadratic fields is exactly the set of fundamental discriminants.
Any prime number p gives rise to an ideal pOK in the ring of integers OK of a quadratic field K. In line with general theory of splitting of prime ideals in Galois extensions, this may be
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