Polynomial ring

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In mathematics, especially in the field of abstract algebra, a polynomial ring is a ring formed from the set of polynomials in one or more variables with coefficients in a ring. Polynomial rings have influenced much of mathematics, from the Hilbert basis theorem, to the construction of splitting fields, and to the understanding of a linear operator. Many important conjectures, such as Serre's conjecture, have influenced the study of other rings, and have influenced even the definition of other rings, such as group rings and rings of formal power series.

[edit] Polynomials in one variable over a field

[edit] Polynomials

A polynomial in X with coefficients in a field K is an expression of the form

$p = p_m X^m + p_{m - 1} X^{m - 1} + \cdots + p_1 X + p_0,$

where p₀, …, p_m are elements of K, the coefficients of p, and X, X ², … are formal symbols ("the powers of X"). Such expressions can be added and multiplied, and then brought into the same form using the ordinary rules for manipulating algebraic expressions, such as associativity, commutativity, distributivity, and collecting the similar terms. Any term p_kX ^k with zero coefficient, p_k = 0, may be omitted. The product of the powers of X is defined by the familiar formula

$X^k\, X^l = X^{k+l},$

where k and l are any natural numbers. Two polynomials are considered to be equal if and only if the corresponding coefficients for each power of X are equal. By convention, X ¹ = X, X ⁰ = 1, and the sum defining the polynomial p may be viewed as the linear combination of the symbols X ^m, …, X ¹, X ⁰ with coefficients p_m, …, p₁, p₀. Using the summation symbol, the same polynomial is expressed more compactly as follows:

$p = p_m X^m + p_{m - 1} X^{m - 1} + \cdots + p_1 X + p_0 = \sum_{k=0}^m p_k X^k.$

The summation limits are frequently omitted, so that the same polynomial is written as

$p = \sum_k p_kX^k,\,$

and it is understood that only finitely many terms are present, i.e. p_k is zero for all large enough values of k, in our case, for k > m. The degree of a polynomial is the largest k such that the coefficient of X ^k is not zero. In the special case of zero polynomial, all of whose coefficients are zero, the degree is undefined, or sometimes defined to be the symbol −∞ ("negative infinity").

[edit] The polynomial ring

The set of all polynomials with coefficients in K forms a commutative ring denoted K[X] and called the ring of polynomials over K. The symbol X is commonly called the "variable", and this ring is also called the ring of polynomials in one variable over K, to distinguish it from more general rings of polynomials in several variables. This terminology is suggested by the important cases of polynomials with real or complex coefficients, which may be alternatively viewed as real or complex polynomial functions. However, in general, X and its powers, X ^k, are treated as formal symbols, not as elements of the field K. One can think of the ring K[X] as arising from K by adding one new element X that is external to K and requiring that X commute with all elements of K. In order for K[X] to form a ring, all powers of X have to be included as well, and this leads to the definition of polynomials as linear combinations of the powers of X with coefficients in K.

A ring has two binary operations, addition and multiplication. In the case of the polynomial ring K[X], these operations are explicitly given by the following formulas:

$\left(\sum_{i=0}^na_iX^i\right) + \left(\sum_{i=0}^n b_iX^i\right) = \sum_{i=0}^n(a_i+b_i)X^i$

and

$\left(\sum_{i=0}^n a_iX^i\right) \cdot \left(\sum_{j=0}^m b_jX^j\right) = \sum_{k=0}^{m+n}\left(\sum_{i + j = k}a_i b_j\right)X^k.$

In the first formula, one of the polynomials may be extended by adding "dummy terms" with zero coefficients, so that the same set of powers formally appears in both summands. In the second formula, the inner summation in the right hand side is only extended over indices within bounds, 0 ≤ i ≤ m and 0 ≤ j ≤ n. Alternative forms of expressing addition and multiplication, without using explicit bounds in the sums, are as follows:

$\left(\sum_i a_iX^i\right) + \left(\sum_i b_iX^i\right) = \sum_i (a_i+b_i)X^i$

and

$\left(\sum_i a_iX^i\right) \cdot \left(\sum_j b_jX^j\right) = \sum_k \left(\sum_{i,j: i + j = k} a_i b_j\right)X^k.$

Since only finitely many coefficients a_i and b_j are non-zero, all sums in effect have only finitely many terms, and hence represent polynomials from K[X]. More generally, the field K can by replaced by any commutative ring R, giving rise to the polynomial ring over R , which is denoted R[X].

[edit] Properties

The polynomial ring K[X] is remarkably similar to the ring Z of integers in many respects. This analogy and the arithmetic of the ring of polynomials were thoroughly investigated by Gauss and his theory served as a model for development of abstract algebra in the second half of the nineteenth century in the works of Kummer, Kronecker, and Dedekind.

[edit] K[X] is a domain

The first property of the polynomial ring is elementary and says that a product of two non-zero polynomials is also a non-zero polynomial. Indeed, the product of a polynomial p of degree m starting with p_mX ^m, p_m ≠ 0, and a polynomial q of degree n starting with q_nX ⁿ, q_n ≠ 0, is the polynomial pq starting with the term rX ^m+n, where the coefficient r = p_mq_n ≠ 0. Hence pq is a non-zero polynomial of degree m + n. Commutative rings in which the product of any two non-zero elements is non-zero are called integral domains, and thus the polynomial ring K[X] is an integral domain.

[edit] Factorization in K[X]

The next property of the polynomial ring is much deeper. Already Euclid noted that every positive integer can be uniquely factored into a product of primes — this statement is now called the fundamental theorem of arithmetic. The proof is based on Euclid's algorithm for finding the greatest common divisor of natural numbers. At each step of this algorithm, a pair (a, b), a > b, of natural numbers is replaced by a new pair (b, r), where r is the remainder from the division of a by b, and the new numbers are smaller. Gauss remarked that the procedure of division with the remainder can also be defined for polynomials: given two polynomials p and q, where q ≠ 0, one can write

p = u q + r,

where the quotient u and the remainder r are polynomials, the degree of r is less than the degree of q, and a decomposition with these properties is unique. The quotient and the remainer are found using the polynomial long division. The degree of the polynomial plays the role analogous to the size of an integer, and since it cannot decrease indefinitely, eventually, the division will be exact, and the last non-zero remainder is the greatest common divisor of the initial two polynomials. In this way, Gauss was able to simultaneously rigorously prove the fundamental theorem of arithmetic for integers and its generalization to polynomials. Commutative rings which admit an analogue of the Euclidean algorithm are called Euclidean rings, and for them, the unique factorization into prime factors holds, they are factorial rings, also called the unique factorization domains. Thus the polynomial ring K[X] is a factorial ring, which is moreover a Euclidean domain.

Another corollary of the polynomial division with the remainder is the fact that every non-zero, proper ideal I of K[X] is principal, i.e. I consists of the multiples of a single non-zero polynomial f which is the greatest common divisor for all polynomials in I. Thus the polynomial ring K[X] is a principal ideal domain.

[edit] Universal property of the polynomial ring

The ring K[X] of polynomials over K is obtained from K by adjoining one element, X. It turns out that any commutative ring L containing K and generated as a ring by a single element in addition to K can be described using K[X]. In particular, this applies to finite field extensions of K.

Suppose that a commutative ring L contains K and there exists an element θ of L such that the ring L is generated by θ over K. Thus any element of L is a linear combination of powers of θ with coefficients in K. Then there is a unique ring homomorphism φ from K[X] into L which does not effect the elements of K itself (it is the identity map on K) and maps each power of X to the same power of θ. Its effect on the general polynomial amounts to "replacing X with θ":

$\phi(a_m X^m + a_{m - 1} X^{m - 1} + \cdots + a_1 X + a_0) = a_m \theta^m + a_{m - 1} \theta^{m - 1} + \cdots + a_1 \theta + a_0.$

By the assumption, any element of L appears as the right hand side of the last expression for suitable m and elements a₀, …, a_m of K. Therefore, φ is surjective and L is a homomorphic image of K[X]. More formally, let Ker φ be the kernel of φ. It is an ideal of K[X] and by the first isomorphism theorem for rings, L is isomorphic to the quotient of the polynomial ring K[X] by the ideal Ker φ. Since the polynomial ring is a principal ideal domain, this ideal is principal: there exists a polynomial p∈K[X] such that

$L \simeq K[X]/(p).$

A particularly important application is to the case when the larger ring L is a field. Then the polynomial p must be irreducible. Conversely, the primitive element theorem states that any finite separable field extension L/K can be generated by a single element θ∈L and the preceding theory then gives a concrete description of the field L as the quotient of the polynomial ring K[X] by a principal ideal generated by an irreducible polynomial p. As an illustration, the field C of complex numbers is an extension of the field R of real numbers generated by a single element i such that i² + 1 = 0. Accordingly, the polynomial X² + 1 is irreducible over R and

$\mathbb{C} \simeq \mathbb{R}[X]/(X^2+1).$

More generally, given a (not necessarily commutative) ring A containing K and an element a of A that commutes with all elements of K, there is a unique ring homomorphism from the polynomial ring K[X] to A that maps X to a:

$\phi: K[X]\to A, \quad \phi(X)=a.$

This homomorphism is given by the same formula as before, but it is not surjective in general. The existence and uniqueness of such a homomorphism φ expresses a certain universal property of the ring of polynomials in one variable and explains ubiquity of polynomial rings in various questions and constructions of ring theory and commutative algebra.

[edit] The polynomial ring in several variables

[edit] Polynomials

A polynomial in n variables X₁,…, X_n with coefficients in a field K is defined analogously to a polynomial in one variable, but the notation is more cumbersome. For any multi-index α = (α₁,…, α_n), where each α_i is a non-negative integer, let

$X^\alpha = \prod_{i=1}^n X_i^{\alpha_i} = X_1^{\alpha_1}\ldots X_n^{\alpha_n}, \quad p_\alpha = p_{\alpha_1\ldots\alpha_n}\in\mathbb{K}.$

The product X^α is called the monomial of multidegree α. A polynomial is a finite linear combination of monomials with coefficients in K:

p =	∑	p_αX^α,
	α

and only finitely many coefficients p_α are different from 0. The degree of a monomial X^α, frequently denoted |α|, is defined as

$|\alpha| = \sum_{i=1}^n \alpha_i,$

and the degree of a polynomial p is the largest degree of a monomial occurring with non-zero coefficient in the expansion of p.

[edit] The polynomial ring

Polynomials in n variables with coefficients in K form a commutative ring denoted K[X₁,…, X_n], or sometimes K[X], where X is a symbol representing the full set of variables, X = (X₁,…, X_n), and called the polynomial ring in n variables. This ring plays fundamental role in algebraic geometry. Many results in commutative and homological algebra originated in the study of its ideals and modules over this ring.

[edit] Hilbert's Nullstellensatz

Main article: Hilbert's Nullstellensatz

A group of fundamental results concerning the relation between ideals of the polynomial ring K[X₁,…, X_n] and algebraic subsets of Kⁿ originating with David Hilbert is known under the name Nullstellensatz (literally: "theorem on the set of zeros").

(Weak form, algebraically closed field of coefficients). Let K be an algebraically closed field. Then every maximal ideal m of K[X₁,…, X_n] has the form

$m = (X_1-a_1, \ldots, X_n-a_n), \quad a = (a_1, \ldots, a_n) \in \mathbb{K}^n.$

(Strong form). Let k be a field with algebraic closure K, I be an ideal in the polynomial ring k[X₁,…, X_n], and V(I) be the algebraic subset of Kⁿ defined by I. Suppose that f is a polynomial which vanishes at all points of V(I). Then some power of f belongs to the ideal I:

$f^m \in I, {\scriptstyle\textrm{\ \ for\ some\ \ }} m\in \mathbb{N}.$

Using the notion of the radical of an ideal, the conclusion says that f belongs to the radical of I. As a corollary of this form of Nullstellensatz, there is a bijective correspondence between the radical ideals of K[X₁,…, X_n] for an algebraically closed field K and the algebraic subsets of the n-dimensional affine space Kⁿ. It arises from the map

$I \mapsto V(I), \quad I\subset K[X_1,\ldots,X_n], \quad V(I)\subset K^n.$

The prime ideals of the polynomial ring correspond to irreducible subvarieties of Kⁿ.

[edit] Properties of the ring extension R ⊂ R[X]

One of the basic techniques in commutative algebra is to relate properties of a ring with properties of its subrings. The notation R ⊂ S indicates that a ring R is a subring of a ring S. In this case S is called an overring of R and one speaks of a ring extension. This works particularly well for polynomial rings and allows one to establish many important properties of the ring of polynomials in several variables over a field, K[X₁,…, X_n], by induction in n.

[edit] Summary of the results

In the following properties, R is a commutative ring and S = R[X₁,…, X_n] is the ring of polynomials in n variables over R. The ring extension R ⊂ S can be built from R in n steps, by successively adjoining X₁,…, X_n. Thus it to establish each of the properties below, it is sufficient to consider the case n = 1.

If R is an integral domain then the same holds for S.

If R is a unique factorization domain then the same holds for S. The proof is based on the Gauss lemma.

Hilbert's basis theorem: If R is a Noetherian ring, then the same holds for S.

Suppose that R is a Noetherian ring of finite global dimension. Then

$\operatorname{gl}\dim R[X_1,\ldots,X_n] = \operatorname{gl} \dim R + n.$

An analogous result holds for Krull dimension.

[edit] Generalizations

Polynomial rings have be generalized in a great many ways, including polynomial rings with generalized exponents, power series rings, non-commutative polynomial rings, and skew-polynomial rings.

[edit] Generalized exponents

Main article: Monoid ring

A simple generalization only changes the set from which the exponents on the variable are drawn. The formulas for addition and multiplication make sense as long as one can add exponents: Xⁱ·X^j = X^i+j. A set for which addition makes sense (is closed and associative) is called a monoid. The set of functions from a monoid N to a ring R which are nonzero at only finitely many places can be given the structure of a ring known as R[N], the monoid ring of N with coefficients in R. The addition is defined component-wise, so that if c = a+b, then c_n = a_n + b_n for every n in N. The multiplication is defined as the Cauchy product, so that if c = a·b, then for each n in N, c_n is the sum of all a_ib_j where i, j range over all pairs of elements of N which sum to n.

When N is commutative, it is convenient to denote the function a in R[N] as the formal sum:

$\sum_{n \in N} a_n X^n$

and then the formulas for addition and multiplication are the familiar:

$\left(\sum_{n \in N} a_n X^n\right) + \left(\sum_{n \in N} b_n X^n\right) = \sum_{n \in N} (a_n+b_n)X^n$

and

$\left(\sum_{n \in N} a_n X^n\right) \cdot \left(\sum_{n \in N} b_n X^n\right) = \sum_{n \in N} \left( \sum_{i+j=n} a_ib_j\right)X^n$

where the latter sum is taken over all i, j in N that sum to n.

Some authors such as (Lang 2002, II,§3) go so far as to take this monoid definition as the starting point, and regular single variable polynomials are the special case where N is the monoid of non-negative integers. Polynomials in several variables simply take N to be the direct product of several copies of the monoid of non-negative integers.

Several interesting examples of rings and groups are formed by taking N to be the additive monoid of non-negative rational numbers, (Osbourne 2000, §4.4).

[edit] Power series

Main article: Formal power series

Power series generalize the choice of exponent in a different direction by allowing infinitely many nonzero terms. This requires various hypotheses on the monoid N used for the exponents, to ensure that the sums in the Cauchy product are finite sums. Alternatively, a topology can be placed on the ring, and then one restricts to convergent infinite sums. For the standard choice of N, the non-negative integers, there is no trouble, and the ring of formal power series is defined as the set of functions from N to a ring R with addition component-wise, and multiplication given by the Cauchy product. The ring of power series can be seen as the completion of the polynomial ring.

[edit] Non-commutative polynomial rings

Main article: Free algebra

For polynomial rings of more than one variable, the products X·Y and Y·X are simply defined to be equal. A more general notion of polynomial ring is obtained when the distinction between these two formal products is maintained. Formally, the polynomial ring in n non-commuting variables with coefficients in the ring R is the monoid ring R[N], where the monoid N is the free monoid on n letters, also known as the set of all strings over an alphabet of n symbols, with multiplication given by concatenation. Neither the coefficients nor the variables need commute amongst themselves, but the coefficients and variables commute with each other.

Just as the polynomial ring in n variables with coefficients in the commutative ring R is the free commutative R-algebra of rank n, the non-commutative polynomial ring in n variables with coefficients in the commutative ring R is the free associative, unital R-algebra on n generators, which is non-commutative when n > 1.

[edit] Differential and skew-polynomial rings

Main article: Ore extension

Other generalizations of polynomials are differential and skew-polynomial rings.

A differential polynomial ring is formed from a ring R and a derivation δ of R into R. Then the multiplication is extended from the relation X·a = a·X + δ(a). The standard example, called a Weyl algebra, takes R to be a polynomial ring k[t], and X to be the standard polynomial derivative $\tfrac{\partial}{\partial t}$ . One views the elements of R[X] as differential operators on the polynomial ring k[t], with elements f(t) of R=k[t] acting as multiplication, and X acting as the derivative in t. Labelling t = Y, one gets the canonical commutation relation, X·Y − Y·X = 1, making the ring explicitly a Weyl algebra. This is a fundamentally important ring, (Lam 2001, §1,ex1.9).

The skew-polynomial ring is defined for a ring R and a ring endomorphism f of R, multiplication is extended from the relation X·r = f(r)·X to give an associative multiplication that distributes over the standard addition. More generally, one has a homomorphism F from the monoid N into the endomorphism ring of R, and Xⁿ·r = F(n)(r)·Xⁿ, as in (Lam 2001, §1,ex 1.11). Skew polynomial rings are closely related to crossed product algebras.

[edit] See also

[edit] References

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (February 2008)

Lam, Tsit-Yuen (2001), A First Course in Noncommutative Rings, Berlin, New York: Springer-Verlag, ISBN 978-0-387-95325-0
Lang, Serge (2002), Algebra, vol. 211, Graduate Texts in Mathematics, Berlin, New York: Springer-Verlag, MR 1878556, ISBN 978-0-387-95385-4
Osborne, M. Scott (2000), Basic homological algebra, vol. 196, Graduate Texts in Mathematics, Berlin, New York: Springer-Verlag, MR 1757274, ISBN 978-0-387-98934-1

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Polynomial ring

From Wikipedia, the free encyclopedia

Contents

[edit] Polynomials in one variable over a field

[edit] Polynomials

[edit] The polynomial ring

[edit] Properties

[edit] K[X] is a domain

[edit] Factorization in K[X]

[edit] Universal property of the polynomial ring

[edit] The polynomial ring in several variables

[edit] Polynomials

[edit] The polynomial ring

[edit] Hilbert's Nullstellensatz

[edit] Properties of the ring extension R ⊂ R[X]

[edit] Summary of the results

[edit] Generalizations

[edit] Generalized exponents

[edit] Power series

[edit] Non-commutative polynomial rings

[edit] Differential and skew-polynomial rings

[edit] See also

[edit] References

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