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The field of fractions of an integral domain is sometimes denoted by or (), and the construction is sometimes also called the fraction field, field of quotients, or quotient field of . All four are in common usage, but are not to be confused with the quotient of a ring by an ideal , which is a quite different concept.
The function field of the n-dimensional space over a field F is F(x 1, ..., x n), i.e., the field consisting of ratios of polynomials in n indeterminates. The function field of X is the same as the one of any open dense subvariety. In other words, the function field is insensitive to replacing X by a (slightly) smaller subvariety.
Field of fractions; Field trace; Formally real field; Function field of an algebraic variety; Function field sieve; Fundamental theorem of algebra; G. Generic polynomial;
In fact, the fraction fields of the rings of regular functions on any affine open set will be the same, so we define, for any U, K X (U) to be the common fraction field of any ring of regular functions on any open affine subset of X. Alternatively, one can define the function field in this case to be the local ring of the generic point.
Another well-known example is the polynomial X 2 − X − 1, whose roots are the golden ratio φ = (1 + √5)/2 and its conjugate (1 − √5)/2 showing that it is reducible over the field Q[√5], although it is irreducible over the non-UFD Z[√5] which has Q[√5] as field of fractions. In the latter example the ring can be made into an UFD ...
To give a non-example, [4] let k be a field and = [,] [], the subalgebra generated by t 2 and t 3. Then A is not integrally closed: it has the field of fractions k ( t ) {\displaystyle k(t)} , and the monic polynomial X 2 − t 2 {\displaystyle X^{2}-t^{2}} in the variable X has root t which is in the field of fractions but not in A.
For example, the definitions of the integers from the natural numbers, of the rational numbers from the integers, of the real numbers from the rational numbers, and of polynomial rings from the field of their coefficients can all be done in terms of universal properties.
In abstract algebra, a valuation ring is an integral domain D such that for every non-zero element x of its field of fractions F, at least one of x or x −1 belongs to D.. Given a field F, if D is a subring of F such that either x or x −1 belongs to D for every nonzero x in F, then D is said to be a valuation ring for the field F or a place of F.