corner of a ring
Does there exist a subset of a ring which is a ring with a multiplicative identity, but not a subring of ?
Let be a ring without the assumption that has a multiplicative identity. Further, assume that is an idempotent
of . Then the subset of the form is called a corner of the ring .
It’s not hard to see that is a ring with as its multiplicative identity:
- 1.
,
- 2.
,
- 3.
is the additive inverse of in ,
- 4.
, and
- 5.
, with , for any .
If has no multiplicative identity, then any corner of is a proper subset of which is a ring and not a subring of . If has 1 as its multiplicative identity and if is an idempotent, then the is not a subring of as they don’t share the same multiplicative identity. In this case, the corner is said to be proper. If we set , then is also a proper corner of .
Remark. If has 1 with an idempotent. Then corners and , where , are direct summands (as modules over ) of via a Peirce decomposition
.
References
- 1 I. Kaplansky, Rings of Operators, W. A. Benjamin, Inc., New York, 1968.