## Section 6.2 Polynomial Rings

Throughout this chapter we shall assume that \(R\) is a commutative ring with identity. Any expression of the form

where \(a_i \in R\) and \(a_n \neq 0\text{,}\) is called a polynomial over \(R\) with indeterminate \(x\text{.}\) The elements \(a_0, a_1, \ldots, a_n\) are called the coefficients of \(f\text{.}\) The coefficient \(a_n\) is called the leading coefficient. A polynomial is called monic if the leading coefficient is 1. If \(n\) is the largest nonnegative number for which \(a_n \neq 0\text{,}\) we say that the degree of \(f\) is \(n\) and write \(\deg f(x) = n\text{.}\) If no such \(n\) existsâ€”that is, if \(f=0\) is the zero polynomialâ€”then the degree of \(f\) is defined to be \(-\infty\text{.}\) We will denote the set of all polynomials with coefficients in a ring \(R\) by \(R[x]\text{.}\) Two polynomials are equal exactly when their corresponding coefficients are equal; that is, if we let

then \(p(x) = q(x)\) if and only if \(a_i = b_i\) for all \(i \geq 0\text{.}\)

To show that the set of all polynomials forms a ring, we must first define addition and multiplication. We define the sum of two polynomials as follows. Let

Then the sum of \(p(x)\) and \(q(x)\) is

where \(c_i = a_i + b_i\) for each \(i\text{.}\) We define the product of \(p(x)\) and \(q(x)\) to be

where

for each \(i\text{.}\) Notice that in each case some of the coefficients may be zero.

###### Example 6.8.

Suppose that

and

are polynomials in \({\mathbb Z}[x]\text{.}\) If the coefficient of some term in a polynomial is zero, then we usually just omit that term. In this case we would write \(p(x) = 3 + 2 x^3\) and \(q(x) = 2 - x^2 + 4 x^4\text{.}\) The sum of these two polynomials is

The product,

can be calculated either by determining the \(c_i\)s in the definition or by simply multiplying polynomials in the same way as we have always done.

###### Example 6.9.

Let

be polynomials in \({\mathbb Z}_{12}[x]\text{.}\) The sum of \(p(x)\) and \(q(x)\) is \(7 + 4 x^2 + 3 x^3 + 4 x^4\text{.}\) The product of the two polynomials is the zero polynomial. This example tells us that we can not expect \(R[x]\) to be an integral domain if \(R\) is not an integral domain.

###### Theorem 6.10.

Let \(R\) be a commutative ring with identity. Then \(R[x]\) is a commutative ring with identity.

###### Proof.

Our first task is to show that \(R[x]\) is an abelian group under polynomial addition. The zero polynomial, \(f(x) = 0\text{,}\) is the additive identity. Given a polynomial \(p(x) = \sum_{i = 0}^{n} a_i x^i\text{,}\) the inverse of \(p(x)\) is easily verified to be \(-p(x) = \sum_{i = 0}^{n} (-a_i) x^i = -\sum_{i = 0}^{n} a_i x^i\text{.}\) Commutativity and associativity follow immediately from the definition of polynomial addition and from the fact that addition in \(R\) is both commutative and associative.

To show that polynomial multiplication is associative, let

Then

The commutativity and distribution properties of polynomial multiplication are proved in a similar manner. We shall leave the proofs of these properties as an exercise.

###### Proposition 6.11.

Let \(p(x)\) and \(q(x)\) be polynomials in \(R[x]\text{,}\) where \(R\) is an integral domain. Then \(\deg p(x) + \deg q(x) = \deg( p(x) q(x) )\text{.}\) Furthermore, \(R[x]\) is an integral domain.

###### Proof.

Suppose that we have two nonzero polynomials

and

with \(a_m \neq 0\) and \(b_n \neq 0\text{.}\) The degrees of \(p(x)\) and \(q(x)\) are \(m\) and \(n\text{,}\) respectively. The leading term of \(p(x) q(x)\) is \(a_m b_n x^{m + n}\text{,}\) which cannot be zero since \(R\) is an integral domain; hence, the degree of \(p(x) q(x)\) is \(m + n\text{,}\) and \(p(x)q(x) \neq 0\text{.}\) Since \(p(x) \neq 0\) and \(q(x) \neq 0\) imply that \(p(x)q(x) \neq 0\text{,}\) we know that \(R[x]\) must also be an integral domain.

We also want to consider polynomials in two or more variables, such as \(x^2 - 3 x y + 2 y^3\text{.}\) Let \(R\) be a ring and suppose that we are given two indeterminates \(x\) and \(y\text{.}\) Certainly we can form the ring \((R[x])[y]\text{.}\) It is straightforward but perhaps tedious to show that \((R[x])[y] \cong R([y])[x]\text{.}\) We shall identify these two rings by this isomorphism and simply write \(R[x,y]\text{.}\) The ring \(R[x, y]\) is called the ring of polynomials in two indeterminates \(x\) and \(y\) with coefficients in \(R\text{.}\) We can define the ring of polynomials in \(n\) indeterminates with coefficients in \(R\) similarly. We shall denote this ring by \(R[x_1, x_2, \ldots, x_n]\text{.}\)

###### Theorem 6.12.

Let \(R\) be a commutative ring with identity and \(\alpha \in R\text{.}\) Then we have a ring homomorphism \(\phi_{\alpha} : R[x] \rightarrow R\) defined by

where \(p( x ) = a_n x^n + \cdots + a_1 x + a_0\text{.}\)

###### Proof.

Let \(p(x) = \sum_{i = 0}^n a_i x^i\) and \(q(x) = \sum_{i = 0}^m b_i x^i\text{.}\) It is easy to show that \(\phi_{\alpha}(p(x) + q(x)) = \phi_{\alpha}(p(x)) + \phi_{\alpha}(q(x))\text{.}\) To show that multiplication is preserved under the map \(\phi_{\alpha}\text{,}\) observe that

The map \(\phi_{\alpha} : R[x] \rightarrow R\) is called the evaluation homomorphism at \(\alpha\text{.}\)

### Reading Questions Reading Questions

###### 1.

Consider the polynomial \(1+3x^4 - 2x\text{.}\)

Is the polynomial monic?

What is the leading coefficient?

What is the degree of the polynomial?

###### 2.

If \(p(x)\) and \(q(x)\) are polynomials in \(R[x]\) for a commutative ring \(R\text{,}\) what can we say about the degree of the polynomial \(p(x)q(x)\text{?}\) Be as specific as possible.

###### 3.

What is the evaluation homomorphism? Illustrate with an example using \(R = \Z\) and \(\alpha = 2\text{.}\)

###### 4.

After reading the section, what questions do you still have? Write at least one well formulated question (even if you think you understand everything).

### Exercises Practice Problems

###### 1.

List out all polynomials of degree 2 in the ring \(\Z_2[x]\text{.}\)

###### 2.

How many polynomials of degree 2 are there in \(\Z_3[x]\text{?}\) Explain.

###### 3.

Are there polynomials of degree 4 in \(\Z_3[x]\text{?}\) Explain.

###### 4.

Consider the two polynomials \(p(x) = 5x^2 + 3x + 6\) and \(q(x) = 4x^2 - x + 9\text{.}\)

Compute \(p(x)+q(x)\) and \(p(x)q(x)\) in the ring \(\Z[x]\text{.}\)

Compute \(p(x)+q(x)\) and \(p(x)q(x)\) in the ring \(\R[x]\text{.}\)

Compute \(p(x)+q(x)\) and \(p(x)q(x)\) in the ring \(\Z_{12}[x]\text{.}\)

### Exercises Collected Homework

###### 1.

Suppose \(p(x)\) and \(q(x)\) are polynomials in \(R[x]\) where \(R\) is a commutative ring with unity. We want to understand how the degree of \(p(x) + q(x)\) and \(p(x)q(x)\) relate to the degrees of \(p(x)\) and \(q(x)\text{.}\)

Give an example of polynomials \(p(x)\) and \(q(x)\) in \(\Z[x]\) so that \(\deg(p(x) + q(x)) \lt \max(\deg p(x),\deg q(x))\text{.}\)

Give an example of polynomials \(p(x)\) and \(q(x)\) in \(\Z_{12}[x]\) so that \(\deg(p(x)q(x)) \lt \deg p(x) + \deg q(x)\text{.}\)

Can you repeat the previous part if \(p(x)\) and \(q(x)\) were polynomials in \(\Z_{11}[x]\text{?}\) Explain.

What can you say in general? In particular, in what situations can you claim that \(\deg(p(x)+q(x)) = \max(\deg p(x), \deg q(x))\) and in what situations can you claim that \(\deg(p(x)q(x)) = \deg p(x) + \deg q(x)\text{?}\)