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1 Math /Foundations of Algebra/Fall 2017 Polynomials at the Foundations: Rational Coefficients The rational numbers are our first field, meaning that all the laws of arithmetic hold, every number has an additive inverse and every number except the additive identity (zero) has a multiplicative inverse. We explore here the arithmetic of polynomials with coefficients in the field of rational numbers and draw a series of close analogies with the arithmetic of the integers. Definition 7.1. Q[x] is the set of polynomials with coefficients in Q. Each polynomial f(x) Q[x] except for f(x) = 0 has the form: f(x) = r d x d + r d 1 x d r 0 where the coefficients r 0,..., r d are rational numbers and r d 0. Definition 7.2. The degree of f(x) = r d x d + + r 0 is d. Remark. Polynomials of degree 0 are also called (non-zero) constants, polynomials of degrees 1, 2 and 3 are called linear, quadratic and cubic, respectively. The zero polynomial f(x) = 0 is the only polynomial not to fit the pattern, and its degree is undefined. The structure of a polynomial resembles that of a natural number. Natural Number Polynomial Digits Coefficients 10 d s place x d so, for instance: 1 3 x3 + 2x 1 2 = 1 3 x3 + 0x 2 + 2x 1 2 is analogous to a four-digit number with digits 1/3, 0, 2 and 1/2. Remark. We lose the bracket notation and refer to the rational number [1/3] as 1/3, always remembering the naming problem: 1/3 = 2/6 =... Addition of polynomials is completely determined by: r d x d + s d x d = (r d + s d )x d In other words, addition is done place by place with no carrying. Example. 1 2 x3 + 3x 2 + 0x x2 2x x x2 2x

2 Proposition 7.3. (a) Addition of polynomials is associative and commutative. (b) The zero polynomial f(z) = 0 is the additive identity, and (c) The additive inverse of f(x) is f(x). Remark. This all immediately follows from the arithmetic of Q. Multiplication of polynomials is completely determined by: (r d x d )(s e x e ) = (r d s e )x d+e and the distributive law. Once again, this is exactly analogous to the multiplication of many-digit numbers, but with no carrying. Example. x 2 x + 1 2x 1 x 2 + x 1 2x 3 2x 2 + 2x 2x 3 3x 2 + 3x 1 Proposition 7.4. (a) Multiplication of polynomials is associative, commutative and distributes with addition. (b) The constant polynomial f(x) = 1 is the multiplicative identity. (c) The product of two polynomials satisfies: deg(f(x)g(x)) = deg(f(x)) + deg(g(x)) (d) The non-zero constant polynomials are the only polynomials with multiplicative inverses. Proof. (a) and (b) follow from the fact that Q is a field. For (c), if f(x) = r d x d + + r 0 and g(x) = s e x e + + s 0, then f(x)g(x) = (r d s e )x d+e + terms with lower place values so the degree of f(x)g(x) is d + e provided that r d s e 0. But since r d 0 and s e 0, it follows that they both have multiplicative inverses (Q is a field!) and (1/r d )(1/s e )(r d s e ) = 1, so r d s e 0. For (d), suppose g(x) is the multiplicative inverse of f(x). Then f(x) g(x) = 1, so by (c), deg(f(x))+deg(g(x)) = 0 and then it follows that deg(f(x)) = 0 and deg(g(x)) = 0, since there are no polynomials with negative degrees.

3 Remark. In particular, (c) shows that f(x) g(x) can only be the zero polynomial if f(x) = 0 or g(x) = 0. Definition 7.5. (a) A set together with addition and multiplication that satisfies all commutative, associative and distributive laws, has an additive identity (0), a multiplicative identity (1) and additive inverses (but not necessarily multiplicative inverses) to all elements is called a Commutative Ring. (b) The elements of a commutative ring with multiplicative inverses are called the units of the ring. Example. Z, Q and Q[x] are all commutative rings with 1. N is not. (a) The units of Z are 1 and 1. (b) Every element of Q except for 0 is a unit. (c) The units of Q[x] are the constant (non-zero) polynomials. Division with Remainders is also a feature of Q[x]. Namely, given f(x) and g(x), of degrees d > e, respectively, then there is a quotient polynomial q(x) such that either: (i) f(x) = q(x)g(x), in which case we say g(x) divides f(x), or else (ii) f(x) = q(x)g(x) + r(x) for a remainder r(x) with deg(r(x)) < e. Proof. Suppose f(x) = r d x d + r 0 and g(x) = s e x e + s 0. We prove this with the long division algorithm. Initialize. Set q(x) = (r d /s e )x d e and let r(x) = f(x) q(x)g(x). Loop. If deg(r(x)) < e (or r(x) = 0), STOP. Otherwise, r(x) = t f x f t 0 so add (t f /s e )x f e to q(x) and reset r(x) = f(x) q(x)g(x). REPEAT. Remark. This is exactly the long division algorithm for polynomials, and it is a perfect analogue of the (harder!) long division algorithm used for dividing natural numbers with many digits. This one is easier because the terms of the polynomial q(x) are so easy to find. Examples. (a) x + 1 does not divide x since: has a non-zero remainder. x = (x 1)(x + 1) + 2 (b) x + 1 does divide x since: has no remainder. x = (x 2 x + 1)(x + 1)

4 Euclid s Algorithm can now be applied to polynomials in Q[x]. Definition 7.6. Any polynomial of maximum degree that divides both f(x) and g(x) is called a gcd of f(x) and g(x). Remark. If h(x) is a gcd of f(x) and g(x), then u h(x) is another gcd, whenever u is any unit polynomial (non-zero constant) because: h(x) = (1/u)(u h(x)) (so u h(x) divides h(x), and thus also divides f(x) and g(x)). We will see that this is the only ambiguity in the gcd. Example. x 1 is a gcd of x 2 1 and x 3 1, but so are: 2x 2, x + 1, 1 2 x + 1 2, etc Euclid s Algorithm and Extras. The following algorithm produces a gcd h(x) of f(x) and g(x) and also solves the equation: a(x)f(x) + b(x)g(x) = h(x) It is exactly the same as Euclid s algorithm for natural numbers! Initialize. Set f 0 (x) = f(x) and g 0 (x) = g(x) (to be fixed throughout). Initialize two equations: a 1 (x)f 0 (x) + b 1 (x)g 0 (x) = f(x) a 2 (x)f 0 (x) + b 2 (x)g 0 (x) = g(x) by initializing a 1 (x) = 1, b 1 (x) = 0, a 2 (x) = 0, b 2 (x) = 1. Loop. Use division with remainders to solve: g(x) = q(x)f(x) + r(x) If r(x) = 0, STOP. Return f(x) and a 2 (x)f 0 (x) + b 2 (x)g 0 (x) = f(x). OTHERWISE, update the two equations via: (a 2 (x) q(x)a 1 (x))f 0 (x) + (b 2 (x) q(x)b 1 (x))g 0 (x) = r(x) a 1 (x)f 0 (x) + b 1 (x)g 0 (x) = f(x) and reset g(x) := f(x) and f(x) := r(x). REPEAT. Remark. Since the degrees of the remainder polynomials decrease with each iteration of the loop, Euclid s algorithm eventually terminates. The argument for why it terminates in a gcd is the same as with the integers. But that argument shows more, namely: Corollary 7.6. Every polynomial that divides both f(x) and g(x) divides the output of Euclid s algorithm.

5 As a consequence, if h(x) is the output of Euclid s algorithm, and d(x) is another gcd, which is necessarily of the same degree, then d(x)k(x) = h(x) and then by Proposition 7.4, k(x) is a constant polynomial, i.e. a unit. Example. Run Euclid s enhanced algorithm on x 5 1 and x 7 1 Initialize: f 0 (x) = x 5 1, g 0 (x) = x 7 1. First Loop. New linear equations: (1)(x 5 1) + (0)(x 7 1) = x 5 1 (0)(x 5 1) + (1)(x 7 1) = x 7 1 x 7 1 = (x 2 )(x 5 1) + (x 2 1) ( x 2 )(x 5 1) + (1)(x 7 1) = x 2 1 (1)(x 5 1) + (0)(x 7 1) = x 5 1 New g(x) = x 5 1. New f(x) = x 2 1. Second Loop. New linear equations: Final Loop. x 5 1 = (x 3 + x)(x 2 1) + (x 1) (x 5 + x 3 + 1)(x 5 1) + ( x 3 x)(x 7 1) = x 1 ( x 2 )(x 5 1) + (1)(x 7 1) = x 2 1 x 2 1 = (x + 1)(x 1) STOP. x 1 is returned by the algorithm, with: (x 5 + x 3 + 1)(x 5 1) + ( x 3 x)(x 7 1) = x 1 We may continue the analogy: Definition 7.7. A polynomial p(x) Q[x] of positive degree is prime if every divisor of p(x) is either a unit (degree zero) or p(x) times a unit (same degree as p(x)). Examples. (i) Constant polynomials are by definition not prime. (ii) All linear polynomials are prime. If p(x) is linear and: f(x) g(x) = p(x) then deg(f(x)) + deg(g(x)) = deg(p(x)) = 1, so either deg(f(x)) = 0 and f(x) is a unit or else deg(f(x)) = 1 and g(x) is a unit.

6 (iii) A quadratic or cubic is prime if and only if it has no linear factors. We ll see that this is the case if and only if it has no roots. (iv) The polynomial: p(x) = x 4 + 2x = (x 2 + 1)(x 2 + 1) is not prime and it also has no linear factors. Factorization. Each f(x) Q[x] is a finite product of primes p(x). Proof. Consider the set S of degrees of non-constant polynomials that are not finite products of primes. Then S is either empty or else S has a smallest element. The smallest element is not 1, since every linear polynomial is prime. Suppose the smallest element of S is n and f(x) is a polynomial of degree n that is not a finite product of primes. Then f(x) is not prime, so it must be a product of two polynomials: g(x)h(x) = f(x) of degrees smaller than n (and adding to n). But then by assumption the degrees of g(x) and h(x) are not in S, so g(x) and h(x) must be finite products of prime polynomials, and we have a contradiction. What about Euler s Theorem? This is a little tricky. Notice that there are infinitely many prime polynomials already in degree one, since each of the polynomials: x r, r Q is prime. Maybe instead we want primes of arbitrarily large degree. In that case we would use the argument from Euler s theorem to take products of primes and add 1: p 1 (x) p n (x) + 1 to get a new polynomial. But unfortunately, this polynomial might factor as a product of linear polynomials, and deny us any new primes. We will see later, however, that the polynomials: x p 1 + x p are always prime whenever p is a prime number! We do have: Unique Factorization. The prime factorization of f(x) is unique up to reordering the primes and multiplying each by a unit. Example. x 2 + 3x + 2 = (x + 1)(x + 2) = ( 1 2 x + 1 ) (2x + 4) 2

7 Finally, we can define the field of rational functions as the set of equivalence classes of polynomial fractions: f(x) g(x) where (f(x)), g(x)) Q[x] Q[x] is an ordered pair of polynomials, and g(x) is not the zero polynomial. Then we define f 1 (x) g 1 (x) f 2(x) g 2 (x) if f 1 (x)g 2 (x) = f 2 (x)g 1 (x) and check by hand that this is an equivalence relation (in the case of integer fractions, this was done for us via the partition of U Z Z). Proposition 6.2 in this setting gives well-defined operations: [ ] [ ] [ ] f1 (x) f2 (x) f1 (x)g 2 (x) + f 2 (x)g 1 (x) + = g 1 (x) g 2 (x) g 1 (x)g 2 (x) and [ ] [ ] [ ] f1 (x) f2 (x) f1 (x)f 2 (x) = g 1 (x) g 2 (x) g 1 (x)g 2 (x) (we will suppress the brackets from now on). The set of equivalence classes of such fractions with this arithmetic is the field of rational functions (with rational coefficients), written: Q(x) Notice also that elements of Q(x) can be chosen in lowest terms, in which f(x) and g(x) share no common prime factors. If the leading coefficient of g(x) is chosen to be 1, then this is uniquely determined. The inductively-minded reader may observe that we took a field, made polynomials with coefficients in that field to create a commutative ring analogous to the integers, in which unique factorization held, and then took fractions of such polynomials to create a new field. This process could be iterated to produce more and more fields: Q, Q(x), Q(x)(y), Q(x)(y)(z),... We will not pursue this particular string of fields beyond Q(x), though we will apply this process to other fields.

8 Exercises Multiply the following polynomials: (a) (x 2 + mx + 1)(x 2 mx + 1) (b) (x n + x n )(x 1) (c) (x 2n x 2n )(x + 1) 8.2. Use Euclid s algorithm to find a gcd h(x) of: and solve the equation: x 5 + x 2 x + 1 and x 8 1 a(x)(x 5 + x 2 x + 1) + b(x)(x 8 1) = h(x) by using Euclid s algorithm with extras Show that a quadratic polynomial: ax 2 + bx + c, is prime if and only if: b 2 4ac fails to have a rational square root. a, b, c Z 8.4. Prove Unique Factorization into Prime Polynomials.

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