Let be a noncommutative finite ring with multiplicative identity element . Show that if the subring generated by is for each nonzero ideal then is simple.
Solution
To show that is simple, we need to prove that has no non-trivial two-sided ideals. We will proceed by contradiction and use the given condition that the subring generated by is for each nonzero ideal .
1. **Assume is not simple**:
Suppose has a non-trivial ideal such that .
2. Direct sum decomposition:
If can be decomposed as a direct sum of nonzero rings , then and must be cyclic by considering and respectively and applying the given condition. This contradicts the non-commutativity of .
3. Finite ring structure:
Since is finite, it can be decomposed into a direct sum of its -subrings using Bezout's lemma. Given that is indecomposable, its order must be for some .
4. Jacobson radical:
Let be the Jacobson radical of . Since is finite, it is Artinian, and thus is semisimple. If , then is simple because an Artinian ring with a trivial radical is simple.
5. **Case when **:
If , then generates , making a cyclic field . Thus, is the unique maximal ideal of , making a finite local ring.
6. **Properties of **:
Note that . For any such that , is cyclic by the problem condition. If , then because both would give when we quotient by them. However, for Artinian rings , any right -module satisfies . Thus, .
7. Contradiction:
Since is finite and Noetherian, it has a proper maximal submodule, leading to a contradiction. Therefore, .
8. Ideal structure:
We claim . By the problem condition, is cyclic unless , in which case and is the field . Since every element of has order , we have , and , so the ideals are equal.
9. Cyclic structure:
Each is cyclic for all such that . If , then . If is the smallest positive integer for which , we can inductively show that for all , .
10. Contradiction in ideal structure:
If has elements, then has elements, and has order . For any nonzero ideal satisfying , we have . Distinct ideals and must be disjoint, leading to . If two distinct such ideals exist, we have , implying . This would make commutative, contradicting the non-commutativity of .
11. Conclusion:
There exists at most one ideal between and . The ideals of form a linear total order under inclusion if . If is nonzero, is an algebra on a free module with basis . If the set is nonempty, where , contradicting that is a cyclic field. Thus, if is nonzero, contradicting the non-commutativity of .
12. Final conclusion:
Therefore, , and since is Artinian with a trivial radical and is indecomposable, is simple.