Notation. Throughout this post, is a field, is a -vector space, and
See that is a two-sided ideal of
If then the ring of matrices with entries in and thus is a simple ring, i.e. the only two-sided ideals of are the trivial ones: and But what if What can we say about the two-sided ideals of if
Theorem 1. If is countably infinite, then is the only non-trivial two-sided ideal of
Proof. Let be a two-sided ideal of and consider two cases.
Case 1. So there exists such that Let be a basis for and let be a subspace of such that Note that is also countably infinite dimensional because Let be a basis for Since the elements are -linearly independent and so we can choose such that for all Now let be such that for all Then and so
Case 2. Choose and suppose that Let be a basis for and extend it to a basis for Since there exists such that Let and fix an such that Now let and suppose that Let be a basis for and for every put For every define as follows: and for all and for all See that and so
Definition. Let be an integer. A ring with unity is called -simple if for every there exist such that
Remark 1. Every -simple ring is simple. To see this, let be a two-sided ideal of and let Then, by definition, there exist such that But, since is a two-sided ideal of we have for all and so
It is not true however that every simple ring is -simple for some For example, it can be shown that the first Weyl algebra is not -simple for any
Theorem 2. If then is -simple. If is countably infinite, then is -simple.
Proof. If then and so we only need to show that is -simple. So let and suppose that is the standard basis for Since there exists such that Using it is easy to see that where on the right-hand side is the identity matrix. This proves that is -simple. If is countably infinite, then, as we proved in Theorem 1, for every there exist such that That means is -simple.
Remark 2. An -simple ring is not necessarily artinian. For example, if is countably infinite, then the ring is -simple but not artinian.
How can I prove the remark 2? It says: An n-simple ring is not necessarily artinian. For example, if $\dim_k$ V is countably infinite, then the ring E/$\mathfrak{I}$ is 1-simple but not artinian. Is this ring noetherian?