Find an Dedekind finite ring that is not IC (other than examples 5.10 & 5.12 given in K.R. Goodearl - Von Neumann Regular Rings)
Solution:
Proof.
Let D be any noncommutative division ring (e.g., Hamilton's ring of real quaternions), and let , where . Since D[x] is a PRID (principal right ideal ring), all modules in are free. This implies that D[x] is stably IC, so R is IC. Since
it follows that R is in fact stably IC. But
...(i)
and by a result of Ojanguren and Sridharan Lemma, the polynomial ring S:=D[x, y] has a non-principal right ideal P such that . This implies that is not internally cancellable. Since , the isomorphism in (i) implies that the ring R[y] is not IC. (Of course, for n=1, R[y]=D[x, y] would have been IC, since it is a domain.)
Thus, There exists a stably IC ring R such that the polynomial ring R[y] is not IC.
Proved.
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