Polynomial ring

In abstract algebra, a polynomial ring is the set of polynomials in one or more variables with coefficients in a ring.

Definition of a polynomial

In real analysis, a polynomial is a certain type of a function of one or several variables (see polynomial), or in other words, a polynomial function.

This definition cannot be adapted to a general ring, however. For example, over the ring Z/2Z of integers modulo 2, the polynomial

P(X)=X²+X=X(X+1)

takes only the value 0, as when k is an integer, k(k+1) is always even. But we would expect P(X) to be different than the zero polynomial.

The approach taken is then the following. Let R be a ring. A polynomial P(X) is defined to be a formal expression of the form

where the coefficients a₀, ..., a_m are elements of the ring R, and X is considered to be a formal symbol. Two polynomials are considered to be equal if and only if the corresponding coefficients for each power of X are equal. Polynomials with coefficients in R can be added by simply adding the corresponding coefficients and multiplied using the distributive law, and the rules

for all elements a of the ring R and

for all natural numbers k and l.

The polynomial ring

One can then check that the set of all polynomials with coefficients in the ring R forms itself a ring, the polynomial ring over R, which is denoted by R[X]. If R is commutative, then R[X] is an algebra over R.

One can think of the ring R[X] as arising from R by adding one new element X to R and only requiring that X commute with all elements of R. In order for R[X] to form a ring, all sums of powers of X have to be included as well.

The polynomial ring in several variables

Given two variables X and Y, one constructs the polynomial ring R[X], and then, on top of it, the ring (R[X])[Y]. This ring is considered the polynomial ring in the two variables R[X,Y].

For example, the polynomial

P(X,Y) = X²Y² + 4XY² + 5X³ - 8Y² + 6XY - 2Y + 7

is thought of as the polynomial

(X² + 4X - 8)Y² + (6X - 2)Y + (5X³ + 7)

in Y with coefficients in R[X].

In similar fashion, the ring R[X₁, ..., X_n] in n variables X₁, ..., X_n is constructed.

Properties

• If R is a field, then R[X] is a principal ideal domain (and even a Euclidean domain).
• If R is a unique factorization domain, so is R[X₁, ..., X_n].
• If R is an integral domain, so is R[X₁, ..., X_n].
• If R is Noetherian, then R[X₁, ..., X_n] is Noetherian. This is the Hilbert basis theorem.
• Every commutative ring that is a finitely-generated algebra over a field can be written as a quotient of a polynomial ring.

Some uses of polynomial rings

Factoring out ideals from a polynomial ring is an important tool for constructing new rings out of known ones.

For instance, the clean construction of finite fields involves the use of those operations, starting out with the field of integers modulo some prime number as the coefficient ring R (see modular arithmetic).

An interesting example of a ring obtained by using polynomials is the ring of Frobenius polynomials, where the ring multiplication is given by function composition, rather than by polynomial multiplication.