# 1.5 The Nil and Jacobson Radicals

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1 1.5 The Nil and Jacobson Radicals The idea of a radical of a ring A is an ideal I comprising some nasty piece of A such that A/I is well-behaved or tractable. Two types considered here are the nil and Jacobson radicals, which are intimately connected with prime and maximal ideals respectively. 123

2 Factoring out by the nil and Jacobson radicals yields quotients closely related to integral domains and fields respectively (in a sense which will be made precise). Consider first all nilpotent elements of a ring: Theorem: Let A be a ring and put N = { x A x is nilpotent }. Then N A and A/N has no nonzero nilpotent elements. 124

3 Proof: Let x,y N, a A. Then x m = y n = 0 ( m,n Z + ). Clearly so (ax) m = ( x) m = 0, ax, x N. 125

4 Also (x + y) m+n 1 = m+n 1 i=0 ( ) m + n 1 i x i y m+n 1 i where ( ) m + n 1 i denotes the usual binomial coefficient, and we interpret integer multiples in the usual way. 126

5 If i m then x i = 0. If i < m then m + n 1 i = n + (m i 1) n so that Thus y m+n 1 i = 0. (x + y) m+n 1 = 0, so x+y N, completing the proof that N A. 127

6 Further, if N + a is a nilpotent element of A/N then (N + a) k = N + a k = N ( k Z + ) so a k N, so a kl = (a k ) l = 0 ( l Z + ), which shows a N and N + a = N. Thus all nonzero elements of A/N are not nilpotent, and the theorem is proved. 128

7 The ideal N of the previous theorem is called the nilradical of A, and the last part of the previous theorem says that the nilradical of what is left, after factoring out by the nilradical, is itself trivial. An alternative description is provided by: Theorem: The nilradical of a ring is the intersection of all of the prime ideals of the ring. 129

8 Before proving this, we observe that its importance derives from the following concept: Suppose { A i i I } is a family of rings and consider the ring A = i I A i = { (x i ) i I x i A i i } with coordinatewise operations, called the direct product of the family. 130

9 Call a ring B a subdirect product of this family if there exists a ring homomorphism such that (i) (ii) ψ : B A ψ is injective; and ρ j ψ : B A j is surjective for all j where ρ j : A A j (x i ) i I x j. is the projection mapping 131

10 Think of subdirect products as being close to direct products. Observation: Let B be a ring and { J i i I } be a family of ideals of B. Put J = J i. i I Then J B and B/J is a subdirect product of the family { B/J i i I }. 132

11 Proof of the Observation: The map ψ : B i I B/J i, x (J i + x) i I is easily seen to be a ring homomorphism with kernel kerψ = i I J i = J. Thus B/J = im ψ. (by the Fundamental Homomorphism Theorem). 133

12 Further, clearly the composite B ψ i I B/J i ρ j B/J j is surjective for each j, so im ψ, and hence B/J, is a subdirect product of the family { B/J i i I }, and the Observation is proved. 134

13 Corollary to the Theorem: If A is any ring and N its nilradical, then A/N is a subdirect product of integral domains. Proof: Let { J i i I } be the family of all prime ideals of a ring A. By the Theorem, N = i I J i is the nilradical of A, and by the Observation, A/N is a subdirect product of the family { A/J i i I }, each member of which is an integral domain. 135

14 Corollary: A ring with trivial nilradical is a subdirect product of integral domains. Proof of the Theorem: Let N denote the nilradical and N the intersection of all prime ideals of the ring A. We have to show N = N. Consider a prime ideal P of A and let x N. 136

15 Then x(x k 1 ) = x k = 0 P ( k Z + ), so, since P is prime, x P or x k 1 P. By a simple induction, x P. Thus N P. Hence N N. 137

16 Suppose x A\N, so x is not nilpotent. Put Σ = { I A ( k Z + ) x k I }, which is a poset with respect to. Certainly Σ, since {0} Σ (because x is not nilpotent). It is easy to verify that Zorn s Lemma applies, guaranteeing the existence of a maximal element P of Σ. 138

17 We will show P is prime. Suppose a,b A and ab P. Suppose a,b P, so P P + aa and P P + ba. But P + aa, P + ba A and P is maximal in Σ. Hence P + aa, P + ba Σ, 139

18 so Then x k P + aa, x l P + ba ( k,l Z + ). x k+l = x k x l P + aba A, so P + aba Σ. But ab P, so P + aba = P Σ, a contradiction. 140

19 Hence a P or b P, proving P is prime. In particular x P, so x N. This proves N N, so we conclude N = N, and the Theorem is proved. 141

20 Define the Jacobson radical R of a ring A to be the intersection of all of the maximal ideals of A. Thus A/R is a subdirect product of fields. Clearly, the maximal ideals of A/R have the form M/R where M is a maximal ideal of A, so the Jacobson radical of A/R is just R/R = {R}, the trivial ideal. 142

21 Since all maximal ideals are prime, immediately that we get nilradical Jacobson radical. The nilradical was defined in terms of a membership test for elements (that they be nilpotent). A membership test exists also for the Jacobson radical. Theorem: Let x A. Then x R ( y A) 1 xy is a unit. 143

22 Proof: (= ) some y A. Suppose 1 xy is not a unit for By Zorn s Lemma (an earlier Corollary) 1 xy M for some maximal ideal M. If x R then x M (since R is the intersection of all maximal ideals) 144

23 and so 1 = (1 xy) + xy M, elements of M which is impossible since M A. Hence x R. 145

24 ( =) Suppose x R. Then x M for some maximal ideal M (since R is the intersection of all maximal ideals). By maximality, A = M {x} = { m + xy m M, y A }. 146

25 In particular, 1 = m + xy ( m M)( y A) so 1 xy = m M. If 1 xy is a unit then M = A. But M A, so 1 xy is not a unit. The Theorem is proved. 147

26 Examples: (1) In any integral domain the nilradical is trivial, and in any local ring the Jacobson radical is the unique maximal ideal. Since there are local rings which are integral domains but not fields, the nil and Jacobson radicals need not be equal. (2) Let A = Z. Claim: R = N = {0}. Proof: Consider 0 a Z. 148

27 If 1 3a = 1 then a = 0, whilst if 1 3a = 1 then a = 2/3, both of which are impossible. Hence 1 3a is not a unit, so a R, so R = {0}. Alternatively (since maximal ideals in Z, being a PID, are precisely nonzero prime ideals) R = p prime pz = {0}. 149

28 (3) Let A = F[x] where F is a field. Claim: R = N = {0}. Proof: Consider 0 α F[x]. Then 1 xα is a polynomial of degree 1, so cannot be a unit (units in F[x] being just the nonzero constants). Hence α R, so again R = {0}. 150

29 (4) Let A = F[G], a group ring, where F is a field of prime characteristic p, and G is an abelian p-group. Claim: In this example R = N = { αg g αg = 0 }. Proof: Consider the onto ring homomomorphism φ : A F where αg g α g, 151

30 whose kernel is ker φ = { α g g αg = 0 }. Thus A/ kerφ = F (by the Fundamental Homomorphism Theorem), so ker φ is a maximal ideal of A. Hence R ker φ. 152

31 Consider x = α 1 g α n g n ker φ. Then α α n = 0, and, for some sufficiently high power m = p k, Hence g m i = 1 ( i = 1,...,n). 153

32 x m = (α 1 g α n g n ) m = (α 1 g 1 ) m (α n g n ) m [by the Freshman s Dream (a + b) p = a p + b p in any field of characteristic p ], 154

33 so x m = α m 1 g m α m n g m n = α m α m n = (α α n ) m = 0 m =

34 Thus x N. Hence R ker φ N R, so and the Claim is proved. R = N = ker φ, Exercise: Let A be a finite ring. (1) Prove that if x A then some power of x is idempotent. (2) Verify that if 0 e = e 2 A then 1 e is idempotent so cannot be a unit. (3) Deduce that N = R. 156

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