38 Irreducibility criteria in rings of polynomials


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1 38 Irreducibility criteria in rings of polynomials 38.1 Theorem. Let p(x), q(x) R[x] be polynomials such that p(x) = a 0 + a 1 x a n x n, q(x) = b 0 + b 1 x b m x m and a n, b m 0. If b m is a unit in R then there exist unique polynomials r(x), s(x) R[x] such that p(x) = s(x)q(x) + r(x) and either deg r(x) < deg q(x) or r(x) = 0. Proof. Exercise (or see Hungerford p.158) Definition. If R is a ring and p(x) R[x] then p(x) defines a function p: R R, a p(a) A function of this form is called a polynomial function Note. Different polynomials may define the same polynomial function. E.g. if p(x) = x + 1, q(x) = x are polynomials in Z/2Z[x] then p(x), q(x) define the same function Z/2Z Z/2Z: p(0) = q(0) = 1, p(1) = q(1) = Definition. Let p(x) R[x]. An element a R is a root of p(x) if f(a) =
2 38.5 Proposition. An element a R is a root of p(x) R[x] iff (x a) p(x). Proof. ( ) If (x a) p(x) then p(x) = q(x)(x a) for some q(x) R[x] so p(a) = q(a)(a a) = 0. ( ) Assume that p(a) = 0. By Theorem 38.1 we have p(x) = s(x)(x a) + r(x) where deg r(x) = 0, so r(x) = b for some b R. This gives 0 = p(a) = s(a)(a a) + b Thus b = 0, and so p(x) = s(x)(x a) Corollary. If R is an integral domain and 0 p(x) R[x] is a polynomial of degree n then R[x] has at most n distinct roots in R. Proof. Let a 1,..., a k R be all distinct roots of p(x). By (38.5) we have p(x) = (x a 1 )q 1 (x) for some q 1 (x) R[x]. Also we have 0 = p(a 2 ) = (a 2 a 1 )q 1 (a 2 ) Since R is an integral domain and a 2 a 1 0 we obtain q 1 (a 2 ), and so q 2 (x) = (x a 2 )q 3 (x) for some q 3 (x) R[x]. This gives p(x) = (x a 1 )(x a 2 )q 3 (x) 151
3 By induction we obtain p(x) = (x a 1 )... (x a k )q k (x) for some 0 q k (x) R[x]. This gives deg p(x) = deg((x a 1 )... (x a k )q k (x)) k 38.7 Note. 1) Corollary 38.6 in not true if R is not an integral domain. E.g. if R = Z/6Z and p(x) = x 2 + x then 0, 2, 3 Z/6Z are roots of p(x). 2) Corollary 38.6 is not true is R is a noncommutative ring (even if R is an integral domain). For example, if R = H and p(x) = x then ±i, ±j, ±k H are roots of p(x). Recall: 33.7 Proposition. If p Z is a prime number then the groups of units (Z/pZ) of the ring Z/pZ is a cyclic group of order (p 1). Proof. For a prime q let P q be the Sylow qsubgroup of (Z/pZ). We have (Z/pZ) = q (p 1) It suffices to show that P q is cyclic for ever q (p 1). Let a be an element of the largest order in P q. We will show that P q = a. Let a = q m. If b P q then b = q k for some k m. As a consequence all elements of P q are roots of the polynomial r(x) = x qm P q
4 On the other hand by (38.6) r(x) has at most q m distinct roots in Z/pZ. It follows that P q q m = a Since a P q this shows that a = P q Note. Proposition 33.7 can be generalized as follows: if F is a finite field and F = F {0} is the multiplicative group of units of F then F is a cyclic group. The proof is the same as for F = Z/pZ Proposition. If F is a field and p(x) F[x] is a polynomial such that deg p(x) > 1 and p(x) has a root in F then p(x) is not irreducible in F[x]. Proof. By (38.6) we have p(x) = q(x)(x a) for some q(x) F[x]. Since deg p(x) > 1 we have deg q(x) > 0, so q(x) and (x a) are not units in R[x] Corollary. Let R be a UFD and let K be the field of fractions of R. If p(x) R[x] is a polynomial such that deg p(x) > 1 and p(x) has a root in K then p(x) is not irreducible in R[x]. Proof. By (38.6) p(x) is not irreducible in K[x], so by (37.10) it is also not irreducible in R[x] Proposition (Integral root test). Let R be a UFD, let K be the field of fractions of R and let p(x) R[x] be a polynomial p(x) = a 0 + a 1 x a n x n 153
5 where a n 0. If a K is a root of p(x) then a is of the form a = b/s where b, s R, gcd(b, s) 1, b a 0 and s a n. In particular, in a n = 1 then a R and a a 0. Proof. Exercise Theorem (Eisenstein Irreducibility Criterion). Let R be a UFD. If p(x) = a 0 + a 1 x a n x n is a primitive polynomial in R[x] such that deg p(x) > 0, and b R is an irreducible element b R such that 1) b a n 2) b a i for all i < n 3) b 2 a 0 then p(x) is irreducible in R[x]. Proof. Assume that p(x) is not irreducible in R[x]. Then we have p(x) = q(x)r(x) for some nonunits q(x), r(x) R[x]. deg q(x), deg r(x) > 0. Let q(x) = c 0 + c 1 x c k x k, Since p(x) is primitive we must have r(x) = d 0 + d 1 x d l x l Notice that since b is irreducible it is a prime element of R and so by (32.4) the ideal b is a prime ideal of R. As a consequence R/ b is an integral domain. Consider the canonical epimorphism π : R R/ b and the induced homomorphism of rings of polynomials π : R[x] R/ b [x] 154
6 By assumption on p(x) we have π(p(x)) = π(a n )x n On the other hand we have π(p(x)) = π(q(x)) π(r(x)) Check: since R/ b is an integral domain we must have π(q(x)) = π(c k )x k, π(r(x)) = π(d l )x l In particular π(c 0 ) = π(d 0 ) = 0, so b c 0 and b d 0. On the other hand a 0 = c 0 d 0, so b 2 a 0 which contradicts the assumption on a Example. If p Z is a prime number then q(x) = x n p is an irreducible polynomial in Z[x]. Note: by (37.10) q(x) is also irreducible in Q[x]. This shows in particular that q(x) has no roots in Q, and so that n p is an irrational number for all primes p and all n > Proposition. Let R be an integral domain and let c R. A polynomial p(x) = n i=0 a ix i is irreducible iff the polynomial p(x c) = n i=0 (x c)i is irreducible. Proof. It is enough to notice that the map is an isomorphism of rings. f : R[x] R[x], f(p(x)) = p(x c) 155
7 38.15 Example. Let p Z be a prime number, and let q(x) = x p 1 + x p x + 1 We will show that q(x) is irreducible in Z[x]. We have This gives q(x + 1) = (x + 1)p 1 (x + 1) 1 = (x + 1)p 1 x = x p 1 + q(x) = xp 1 x 1 ( ) p x p ( ) ( ) p p x p p 1 Since p ( ) p k for k = 1,..., p 1 and p 2 ( p p 1) the polynomial q(x + 1) is irreducible in Z[x], and so q(x) is also irreducible. 156
8 39 Modules 39.1 Definition. Let R be a (possibly noncommutative) ring. A left Rmodule is an abelian group M together with a map R M M, (r, m) rm satisfying the following conditions: 1) r(m + n) = rm + rn 2) (r + s)m = rm + sm 3) (rs)m = r(sm) 4) If R is a ring with identity 1 R then 1m = m for all m M. A right Rmodule is defined analogously Definition. If M, N are left Rmodules then a map f : M N is a left Rmodules homomorphism if f is a homomorphism of abelian groups and f(rm) = rf(m) for all r R, m M Note. Left Rmodules and their homomorphisms form a category RMod. Analogously, right Rmodules form a category ModR Examples. 1) If I is a left ideal of R then I is a left module of R. In particular R is a left Rmodule. 2) If F is a field then left (or right ) Fmodules are vector spaces over F, and homomorphisms of Fmodules are Flinear maps. 157
9 3) The category of left (or right) Zmodules is isomorphic to the category of abelian groups: if G is an abelian group then G has a natural Zmodule structure such that for n Z and g G we have ng = n times {}}{ g + + g for n > 0 0 for n = 0 ( g) + + ( g) }{{} n times for n < 0 4) Let G be an abelian group, and let R = Hom(G, G) be the ring of homomorphisms of G (with the usual addition of homomorphims and multiplication given by composition of homomorphisms). We have a map R G G, ϕ g = ϕ(g) Check: this defines a left Rmodule structure on G. Note: the multiplication G R G, g ϕ = ϕ(g) does not define a right module structure on G. Indeed, for ϕ, ψ R we have: (g ψ) ϕ = (ψ(g)) ϕ = ϕ(ψ(g)) One the other hand g (ψ ϕ) = ψ(ϕ(g)). Since in general ψ(ϕ(g)) ϕ(ψ(g)) we get that (g ψ) ϕ g (ψ ϕ) 39.5 Note. For a ring R define a ring R op as follows: R op = R as abelian group r Rop s := sr 158
10 We have: (left Rmodules) = (right R op modules) (check!). In particular, if R is a commutative ring then R = R op, and so (left Rmodules) = (right Rmodules) Note. From now on by an Rmodule we will understand a left Rmodule. 159
11 40 Basic operations on modules 40.1 Definition. If M in an Rmodule then a submodule of M is an additive subgroup N M such that if r R and n N then rn N Note. If f : M N is a homomorphism of Rmodules then Ker(f) := f 1 (0) is a submodule of M and Im(f) := f(m) is a submodule of N Definition. If M is an Rmodule and S is a subset of M then the module generated by S is the submodule S M that is the smallest submodule of M containing S. If S = M then we say that the set S generates M. A module M is finitely generated if M = S for some finite set S Note. If M is an Rmodule and S M then S = {r 1 m r k m k r i R, m i M} 40.5 Definition. If M is an Rmodule and N M is a submodule then the quotient module M/N is the quotient abelian group with multiplication defined by r(m + N) := rm + N for r R, m + N M/N First Isomorphism Theorem. If f : M N is a homomorphism of Rmodules that is onto then M/ Ker(f) = N 160
12 Proof. Similar to the proof of Theorem 6.1 for groups Definition. If {M i } i I is a family of Rmodules then the direct product of {M i } i I is the module M i = {(m i ) i I m i M i } i I with addition and multiplication by R defined coordinatewise. The direct sum of {M i } i I is the submodule i I M i of i I M i given by M i := {(m i ) i I m i 0 for finitely many i only } i I 40.8 Note. Recall the notions of categorical products and copruducts (Section 12). Check: i I M i is the categorical product of the family {M i } i I in the category RMod. i I M i is the categorical coproduct of {M i } i I in the category RMod. 161
13 41 Free modules and vector spaces 41.1 Definition. Let M be an Rmodule. A set S M is linearly independent if for any distinct m 1,..., m k S we have only if r 1 =... = r k = 0. r 1 m r k m k = Definition. Let M be an Rmodule. A set B M is a basis of M if B is linearly independent and B generates M Definition. An Rmodule M is a free module if M has a basis Theorem. Let R be a ring with identity 1 0 and let F be an Rmodule. The following conditions are equivalent. 1) F is a free module. 2) F = i I R for some set I. 3) There is a nonempty subset B F satisfying the following universal property. For any Rmodule M and any map of sets f : B M there is a unique Rmodule homomorphism f : F N such that the following diagram commutes: f B M i f F Here i: B F is the inclusion map. Proof. Exercise. 162
14 41.5 Note. Let U : RMod Set be the forgetfull functor. Check: U has a left adjoint functor F RMod : Set RMod One can show that an Rmodule M is free iff M = F RMod (S) for some set S (exercise) Note. We have: (free Zmodules) = (free abelian groups) 41.7 Theorem. If R is a division ring then every Rmodule is free. Proof. Let M be an Rmodule. It is enough to show that M has a basis. Let S be the set of all linearly independent subsets of M ordered with respect to inclusion of subsets. Claim 1. S has a maximal element. Indeed, by Zorn s Lemma (29.10) it is enough to show that every chain in S has an upper bound. Let then T = {B i } i I be a chain in S. Take B := i I B i. We have B i B for all i I, so it suffices check that B is a linearly independent. Let then b 1,..., b k B, and assume that r 1 b r k b k = 0 We need to show that r 1 =... = r k = 0. We have b 1 B i1,..., b k B ik for some i 1,..., i k I. Since {B i } i I is a chain we can assume that B i1 B i2... B ik As a consequence b 1,..., b k B ik, and since B ik is a linearly independent set we get that r 1 =... = r k =
15 Claim 2. If B is a maximal element in S then B is a basis of M. Indeed, by the definition of S the set B is linearly independent so it is enough to show that B = M. Assume that this is not true, and let m M B. Take the set B = B {m} Notice that the set B is linearly independent. To see this, assume that for some b 1,..., b k B and r 1,... r k, s R we have r 1 b r k b k + sm = 0 If s 0 then s is a unit (since R is a division ring) and so m = ( s 1 r 1 )b ( s 1 r k )b k This is however impossible since m B. Therefore s = 0, and so r 1 b r k b k = 0 Linear independence of B gives then s = r 1 =... = r k = 0 As a consequence we get that B S and B B. This is however a contradiction since B is a maximal element of S Note. Let R be a division algebra and let M be an Rmodule. By a similar argument as in the proof of Theorem 41.7 we can show that: 1) if V M is a linearly independent set then there is a basis B of M such that V B; 2) if V M is a set generating M then then there is a basis B of M such that B V. 164
16 41.9 Note. 1) For a general ring R it is not true that a linearly independent subset V of a free Rmodule F can be always extended to a basis. Take e.g. R = Z, F = Z, V = {2} 2) It is also not true in general that if V is a set generating a free Rmodule then V contains a basis of F. Take e.g. R = Z, F = Z, V = {2, 3} 165
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