# Chapter 13: Basic ring theory

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1 Chapter 3: Basic ring theory Matthew Macauley Department of Mathematical Sciences Clemson University Math 42, Spring 24 M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 / 7

2 Introduction Definition A ring is an additive (abelian) group R with an additional binary operation (multiplication), satisfying the distributive law: x(y + z) = xy + xz and (y + z)x = yx + zx x, y, z R. Remarks There need not be multiplicative inverses. Multiplication need not be commutative (it may happen that xy yx). A few more terms If xy = yx for all x, y R, then R is commutative. If R has a multiplicative identity = R, we say that R has identity, or R is a ring with. A subring of R is a subset S R that is also a ring. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 2 / 7

3 Introduction Examples. Z, Q, R, and C are commutative rings with. 2. Z n is a commutative ring with. 3. For any ring R, the set M n(r) of n n matrices over R is a ring with = I n. 4. For any set X, the set of functions F = {f : X X } is a ring by defining (f + g)(x) = f (x) + g(x) (fg)(x) = f (x)g(x). 5. The set S = 2Z is a subring of Z but it does not have. {[ ] } a 6. S = : a R is a subring of R = M 2(R). However, note that R = [ ], but S = 7. If R is a ring and x a variable, then the set is called the polynomial ring over R. [ ]. R[x] = {a nx n + a x + a a i R} M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 3 / 7

4 Another example: the quaternions Recall the group Q 4 = i, j, k i 2 = j 2 = k 2 =, ij = k. Allowing addition makes them into a ring H, called the quaternions, or Hamiltonians: H = {a + bi + cj + dk a, b, c, d R}. The set H is isomorphic to a subring of M n(r), the real-valued 4 4 matrices: a b c d b a d c H = c d a b : a, b, c, d R M4(R). d c b a Formally, we have an embedding φ: H M 4(R) where [ ] [ ] φ(i) =, φ(j) = We say that H is represented by a set of matrices., φ(k) = [ ]. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 4 / 7

5 Units and zero divisors Definition Let R be a ring with. A unit is any x R that has a multiplicative inverse. Let U(R) be the set (a multiplicative group) of units of R. A nonzero element x R is a left zero divisor if xy = for some y. (Analogously, y is a right zero divisor). Examples. Let R = Z. The units are U(R) = {, }. There are no zero divisors. 2. Let R = Z n. A nonzero k Z n is a unit if gcd(n, k) =, and a zero divisor if gcd(n, k) The ring R = M 2(R) has zero divisors, such as: [ ] [ ] [ ] = The groups of units of M 2(R) are the invertible matrices. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 5 / 7

6 Types of rings Definition If all non-zero elements of R have a multplicitive inverse, then R is a division ring. (Think: field without commutativity.) An integral domain is a commutative ring with no zero divisors. (Think: field without inverses.) A field is just a commutative division ring. Moreover: Examples fields division rings fields integral domains all rings Rings that are not integral domains: Z n (composite n), M n(r), Z Z, H. Integral domains that are not fields (or even division rings): Z, Z[x], R[x], R[[x]] (formal power series). Division ring but not a field: H. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 6 / 7

7 Ideals In the theory of groups, we can quotient out by a subgroup if and only if it is a normal subgroup. The analogue of this for rings are (two-sided) ideals. Definition A subring I R is a left ideal if rx I for all r R and x I. Right ideals, and two-sided ideals are defined similarly. If R is commutative, then all left (or right) ideals are two-sided. We use the term ideal and two-sided ideal synonymously. Examples nz is an ideal of Z. If R = M 2(R), then I = {[ ] } a : a, c R is a left, but not a right ideal of R. c The set Sym n (R) of symmetric n n matrices is a subring of M n(r), but not an ideal. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 7 / 7

8 Ideals Remark If an ideal I of R contains, then I = R. Proof Suppose I, and take an arbitrary r R. Then r I, and so r = r I. Therefore, I = R. It is not hard to modify the above result to show that if I contains any unit, then I = R. (HW) Let s compare the concept of a normal subgroup to that of an ideal: normal subgroups are characterized by being invariant under conjugation: H G is normal iff ghg H for all g G. (left) ideals of rings are characterized by being invariant under (left) multiplication: I R is a (left) ideal iff ri I for all r R. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 8 / 7

9 Ideals The ideal generated by a set S = {r,..., r k } I is defined as (r,..., r k ) := {I : I is an ideal s.t. S I R}. This is the smallest ideal containing S. Since an ideal I of R is an additive subgroup (and hence normal), the quotient R/I = {x + I x R} is an additive group. Proposition If I R is a (two-sided) ideal, then R/I is a ring (called a quotient ring), where multiplication is defined by (x + I )(y + I ) := (xy + I ). Proof We need to show this is well-defined. Suppose x + I = r + I and y + I = s + I. This menas that x r I and y s I. It suffices to show that xy + I = rs + I, or equivalently, xy rs I : xy rs = xy ry + ry rs = (x r)y + r(y s) I. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 9 / 7

10 Finite fields We ve already seen that Z p is a field if p is prime. Let R = Z 2[x] be the polynomial ring over the field Z 2. (So we can ignore all negative signs.) The polynomial f (x) = x 2 + x + is irreducible over Z 2 because it does not have a root. (Note that f () = f () =.) Consider the ideal I = (x 2 + x + ), the set of multiples of x 2 + x +. In the quotient ring R/I, we have the relation x 2 + x + =, or equivalently, x 2 = x = x +. The quotient has only 4 elements: + I, + I, x + I, (x + ) + I. As we do with the quotient group (or ring) Z/nZ, we usually drop the I, and just write R/I = Z 2[x]/(x 2 + x + ) = {,, x, x + }. It is easy to check that this is a field! M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 / 7

11 Finite fields Here is a Cayley diagram, and the operation tables for R/I = Z 2[x]/(x 2 + x + ): + x x + x x + x x + x + x x x + x x + x x + x x + x + x x x + x x + x + x Theorem There exists a finite field F q of order q, which is unique up to isomorphism, iff q = p n for some prime p. If n >, then this field is isomorphic to the quotient ring Z p[x]/(f ), where f is any irreduicible polynomial of degree n. Much of the error correcting techniques in coding theory are built using mathematics over F 2 8 = F 256. This is what allows your CD to play despite scratches. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 / 7

12 Ring homomorphisms Definition A ring homomorphism is a function f : R S satisfying f (x + y) = f (x) + f (y) and f (xy) = f (x)f (y) for all x, y R. A ring isomorphism is a homomorphism that is bijective. The kernel f : R S is the set ker f = {x R : f (x) = }. Examples. The function φ: Z Z n that sends k k (mod n) is a ring homomorphism with ker(φ) = nz. 2. For a fixed real number a R, the evaluation function φ: R[x] R, φ: p(x) p(a) is a homomorphism. The kernel consists of all polynomials that have a as a root. 3. The following is a homomorphism, for the ideal I = (x 2 + x + ) in Z 2[x]: φ: Z 2[x] Z 2[x]/I, f (x) f (x) + I. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 2 / 7

13 The homomorphism theorems for rings Fundamental homomorphism theorem If φ: R S is a ring homomorphism, then ker φ is an ideal and Im(φ) = R/ ker(φ). R (I = ker φ) φ any homomorphism Im φ S quotient process q g remaining isomorphism ( relabeling ) R / ker φ quotient ring Proof The statement holds for the underlying additive group R. Thus, it remains to show that ker φ is an ideal, and the following map is a ring homomorphism (HW): g : R/I Im φ, g(x + I ) = φ(x). M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 3 / 7

14 The homomorphism theorems for rings Freshman theorem Suppose R is a ring with ideals J I. Then I /J is an ideal of R/J and (R/J)/(I /J) = R/I. Diamond isomorphism theorem Suppose I and J are ideals of R. Then (i) The sum I + J = {r + s r I, s J} and intersection I J are ideals of R. (ii) The following quotient rings are isomorphic: (I + J)/I = J/(I J). I R I + J J I J Correspondence theorem Let I be an ideal of R. There is a bijective correspondence between ideals of R/I and ideals of R that contain I. In particular, every ideal of R/I has the form J/I, for some ideal J satisfying I J R. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 4 / 7

15 Maximal ideals Definition An ideal I of R is maximal if I R and if I J R holds for some ideal J, then J = I or J = R. A ring R is simple if its only (two-sided) ideals are and R. Examples. The ideal M = (n) of R = Z is maximal if and only if n is prime. 2. Let R = Q[x] be the set of all polynomials over Q. The ideal M = (x) conisting of all polynomials with constant term zero is a maximal ideal. Elements in the quotient ring Q[x]/(x) have the form f (x) + M = a + M. In both examples above, the quotient R/M is a field. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 5 / 7

16 Maximal ideals Theorem Let R be a commutative ring with. The following are equivalent for an ideal I R. (i) I is a maximal ideal; (ii) R/I is simple; (iii) R/I is a field. Proof The equivalence (i) (ii) is immediate from the Correspondence Theorem. For (ii) (iii), we ll show that an arbitrary ring R is simple iff R is a field. = : Assume R is simple. Then (a) = R for any any nonzero a R. Thus, (a), so = ba for some b R, so a U(R) and R is a field. =: Let I R be a nonzero ideal of a field R. Take a I. Then a a I, and so I, which means I = R. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 6 / 7

17 Prime ideals Definition Let R be a commutative ring. An ideal P R is prime if ab P implies either a P or b P. Note that p N is a prime number iff p = ab implies either a = p or b = p. Examples. The ideal (n) of Z is a prime ideal iff n is a prime number. 2. In the polynomial ring Z[x], the ideal I = (2, x) is a prime ideal. It consists of all polynomials whose constant coefficient is even. Theorem An ideal P R is prime iff R/P is an integral domain. The proof is straightforward (HW). Since fields are integral domains, the following is immediate: Corollary In a commutative ring, every maximal ideal is prime. M. Macauley (Clemson) Chapter 3: Basic ring theory Math 42, Spring 24 7 / 7

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