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Algebra Difficulty 8.3 Shortlist Prove it

Let RR be a noncommutative finite ring with multiplicative identity element 11. Show that if the subring generated by I{1}I \cup \{1\} is RR for each nonzero ideal II then RR is simple.

Solution

To show that R R is simple, we need to prove that R R has no non-trivial two-sided ideals. We will proceed by contradiction and use the given condition that the subring generated by I{1} I \cup \{1\} is R R for each nonzero ideal I I .

1. **Assume R R is not simple**:
Suppose R R has a non-trivial ideal I I such that 0IR 0 \subsetneq I \subsetneq R .

2. Direct sum decomposition:
If R R can be decomposed as a direct sum of nonzero rings AB A \oplus B , then A A and B B must be cyclic by considering I=B I = B and I=A I = A respectively and applying the given condition. This contradicts the non-commutativity of R R .

3. Finite ring structure:
Since R R is finite, it can be decomposed into a direct sum of its p p -subrings using Bezout's lemma. Given that R R is indecomposable, its order must be pk p^k for some k k .

4. Jacobson radical:
Let J(R) J(R) be the Jacobson radical of R R . Since R R is finite, it is Artinian, and thus R/J(R) R/J(R) is semisimple. If J(R)=0 J(R) = 0 , then R R is simple because an Artinian ring with a trivial radical is simple.

5. **Case when J(R)0 J(R) \neq 0 **:
If J(R)0 J(R) \neq 0 , then 1 1 generates R/J(R) R/J(R) , making R/J(R) R/J(R) a cyclic field Fp \mathbb{F}_p . Thus, J(R) J(R) is the unique maximal ideal of R R , making R R a finite local ring.

6. **Properties of J(R) J(R) **:
Note that pJ(R) p \in J(R) . For any k k such that J(R)k0 J(R)^k \neq 0 , R/(J(R)k) R/(J(R)^k) is cyclic by the problem condition. If pJ(R)2 p \in J(R)^2 , then J(R)=J(R)2 J(R) = J(R)^2 because both would give Fp \mathbb{F}_p when we quotient R R by them. However, for Artinian rings R R , any right R R -module M M satisfies MJ(R)=rad(M) M \cdot J(R) = \text{rad}(M) . Thus, J(R)2=rad(J(R)) J(R)^2 = \text{rad}(J(R)) .

7. Contradiction:
Since J(R) J(R) is finite and Noetherian, it has a proper maximal submodule, leading to a contradiction. Therefore, pJ(R)J(R)2 p \in J(R) - J(R)^2 .

8. Ideal structure:
We claim (p)=J(R) (p) = J(R) . By the problem condition, R/(p) R/(p) is cyclic unless p=0 p = 0 , in which case J(R)=0 J(R) = 0 and R R is the field Fp \mathbb{F}_p . Since every element of R/(p) R/(p) has order p p , we have R/(p)=R/J(R)=Fp R/(p) = R/J(R) = \mathbb{F}_p , and (p)J(R) (p) \subseteq J(R) , so the ideals are equal.

9. Cyclic structure:
Each R/(pk) R/(p^k) is cyclic for all k k such that (pk)0 (p^k) \neq 0 . If (pk+1)0 (p^{k+1}) \neq 0 , then (pk)rad(pk)=(pk+1) (p^k) \neq \text{rad}(p^k) = (p^{k+1}) . If q q is the smallest positive integer for which pq=0 p^q = 0 , we can inductively show that for all n<q n < q , R/(pn)=Z/(pn)Z R/(p^n) = \mathbb{Z}/(p^n)\mathbb{Z} .

10. Contradiction in ideal structure:
If (pq1) (p^{q-1}) has k k elements, then R R has pq1k p^{q-1} \cdot k elements, and 1 1 has order pq p^q . For any nonzero ideal U U satisfying 0<U<(pq1) 0 < U < (p^{q-1}) , we have R/U=Z/(pq)Z R/U = \mathbb{Z}/(p^q)\mathbb{Z} . Distinct ideals U1 U_1 and U2 U_2 must be disjoint, leading to U1+U2=(pq1) U_1 + U_2 = (p^{q-1}) . If two distinct such ideals exist, we have k2/p2=k k^2/p^2 = k , implying k=p2 k = p^2 . This would make R R commutative, contradicting the non-commutativity of R R .

11. Conclusion:
There exists at most one ideal between (pq1) (p^{q-1}) and 0 0 . The ideals of R R form a linear total order under inclusion if J(R)0 J(R) \neq 0 . If J(R) J(R) is nonzero, R R is an algebra on a free Z/(pq)Z \mathbb{Z}/(p^q)\mathbb{Z} module with basis 1,b1,,bm 1, b_1, \ldots, b_m . If the set bi b_i is nonempty, (p)=(Z/pZ)1+m (p) = (\mathbb{Z}/p\mathbb{Z})^{1+m} where m>0 m > 0 , contradicting that R/(p)=R/J(R)=Fp R/(p) = R/J(R) = \mathbb{F}_p is a cyclic field. Thus, R=Z/(pq)Z R = \mathbb{Z}/(p^q)\mathbb{Z} if J(R) J(R) is nonzero, contradicting the non-commutativity of R R .

12. Final conclusion:
Therefore, J(R)=0 J(R) = 0 , and since R R is Artinian with a trivial radical and is indecomposable, R R is simple.

\blacksquare

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