Basic Properties of Rings

15. Basic Properties of Rings We first prove some standard results about rings. Lemma 15.1. Let R be a ring and let a and b be elements of R. Then (1) a0 = 0a = 0. (2) a(−b) = (−a)b = −(ab). Proof. Let x = a0. We have x = a0 = a(0 + 0) = a0 + a0 = x + x. Adding −x to both sides, we get x = 0, which is (1). Let y = a(−b). We want to show that y is the additive inverse of ab, that is we want to show that y + ab = 0. We have y + ab = a(−b) + ab = a(−b + b) = a0 = 0, by (1). Hence (2). D Lemma 15.2. Let R be a set that satisfies all the axioms of a ring, except possibly a + b = b + a. Then R is a ring. Proof. It suffices to prove that addition is commutative. We compute (a + b)(1 + 1), in two different ways. Distributing on the right, (a + b)(1 + 1) = (a + b)1 + (a + b)1 =a+b+a+b = a + (b + a) + b. On the other hand, distributing this product on the left we get (a + b)(1 + 1) = a(1 + 1) + b(1 + 1) = a + a + b + b. Thus a + (b + a) + a = (a + b)(1 + 1) = a + a + b + b. 1 MIT OCW: 18.703 Modern Algebra Prof. James McKernan Cancelling an a on the left and a b on the right, we get b + a = a + b, which is what we want. D Note the following identity. Lemma 15.3. Let R be a ring and let a and b be any two elements of R. Then (a + b)2 = a2 + ab + ba + b2 . Proof. Easy application of the distributive laws. D Definition 15.4. Let R be a ring. We say that R is commutative if multiplication is commutative, that is a · b = b · a. Note that most of the rings introduced in the the first section are not commutative. Nevertheless it turns out that there are many interest­ ing commutative rings. Compare this with the study of groups, when abelian groups are not considered very interesting. Definition-Lemma 15.5. Let R be a ring. We say that R is boolean if for every a ∈ R, a2 = a. Every boolean ring is commutative. Proof. We compute (a + b)2 . a + b = (a + b)2 = a2 + ba + ab + b2 = a + ba + ab + b. Cancelling we get ab = −ba. If we take b = 1, then a = −a, so that −(ba) = (−b)a = ba. Thus ab = ba. D Definition 15.6. Let R be a ring. We say that R is a division ring if R − {0} is a group under multiplication. If in addition R is commu­ tative, we say that R is a field. Note that a ring is a division ring iff every non-zero element has a multiplicative inverse. Similarly for commutative rings and fields. Example 15.7. The following tower of subsets Q⊂R⊂C is in fact a tower of subfields. Note that Z is not a field however, as 2 does not have a multiplicative inverse. Further the subring of Q given 2 MIT OCW: 18.703 Modern Algebra Prof. James McKernan by those rational numbers with odd denominator is not a field either. Again 2 does not have a multiplicative inverse. Lemma 15.8. The quaternions are a division ring. Proof. It suffices to prove that every non-zero number has a multiplica­ tive inverse. Let q = a + bi + cj + dk be a quaternion. Let q̄ = a − bi − cj − dk, the conjugate of q. Note that qq̄ = a2 + b2 + c2 + d2 . As a, b, c and d are real numbers, this product if non-zero iff q is non-zero. Thus q̄ p= 2 , 2 a + b + c2 + d2 is the multiplicative inverse of q. D Here is an obvious necessary condition for division rings: Definition-Lemma 15.9. Let R be a ring. We say that a ∈ R, a = 0, is a zero-divisor if there is an element b ∈ R, b = 0, such that, either, ab = 0 or ba = 0. If a has a multiplicative inverse in R then a is not a zero divisor. Proof. Suppose that ba = 0 and that c is the multiplicative inverse of a. We compute bac, in two different ways. bac = (ba)c = 0c = 0. On the other hand bac = b(ac) = b1 = b. Thus b = bac = 0. Thus a is not a zero divisor. D Definition-Lemma 15.10. Let R be a ring. We say that R is a domain if R has no zero-divisors. If in addition R is commutative, then we say that R is an integral domain. Every division ring is a domain. Unfortunately the converse is not true. 3 MIT OCW: 18.703 Modern Algebra Prof. James McKernan Example 15.11. Z is an integral domain but not a field. In fact any subring of a division ring is clearly a domain. Many of the examples of rings that we have given are in fact not domains. Example 15.12. Let X be a set with more than one element and let R be any ring. Then the set of functions from X to R is not a domain. Indeed pick any partition of X into two parts, X1 and X2 (that is suppose that X1 and X2 are disjoint, both non-empty and that their union is the whole of X). Define f : X −→ R, by x ∈ X1 x ∈ X2 , f (x) = 0 1 g(x) = 1 x ∈ X1 0 x ∈ X2 . and g : X −→ R, by Then f g = 0, but neither f not g is zero. Thus f is a zero-divisor. Now let R be any ring, and suppose that n > 1. I claim that Mn (R) is not a domain. We will do this in the case n = 2. The general is not much harder, just more involved notationally. Set A=B= 0 1 . 0 0 Then it is easy to see that 0 0 . 0 0 AB = Note that the definition of an integral domain involves a double negative. In other words, R is an integral domain iff whenever ab = 0, where a and b are elements of R, then either a = 0 or b = 0. 4 MIT OCW: 18.703 Modern Algebra Prof. James McKernan