TSIU-KWEN LEEZhou, Y.Y.Zhou2018-09-102018-09-102012http://www.scopus.com/inward/record.url?eid=2-s2.0-84863240806&partnerID=MN8TOARShttp://scholars.lib.ntu.edu.tw/handle/123456789/369834A ring is called clean (resp., uniquely clean) if each of its elements can be (resp., uniquely) expressed as the sum of an idempotent and a unit. Motivated by recent work on uniquely clean rings in [6 Nicholson , W. K. , Zhou , Y. ( 2004 ). Rings in which elements are uniquely the sum of an idempotent and a unit . Glasg. Math. J. 46 : 227 – 236 .[Crossref], [Web of Science ®] , [Google Scholar]], we introduce the clean index of a ring R. For a ∈ R, let ℰ(a) = {e ∈ R: e 2 = e, a − e ∈ U(R)} where U(R) is the group of units of R and the clean index of R, denoted in(R), is defined by in(R) = sup{|ℰ(a)|: a ∈ R}. Thus, R is uniquely clean if and only if R is clean with in(R) = 1. So far, uniquely clean rings are the only clean rings whose structure is fully understood (see [6 Nicholson , W. K. , Zhou , Y. ( 2004 ). Rings in which elements are uniquely the sum of an idempotent and a unit . Glasg. Math. J. 46 : 227 – 236 .[Crossref], [Web of Science ®] , [Google Scholar]]). In this article, we characterize the (arbitrary) rings of clean indices 1, 2, 3 and determine the abelian rings of finite clean index. Applications to semipotent rings, semiprime rings, and clean rings are discussed.[SDGs]SDG9journal article10.1080/00927872.2010.5387812-s2.0-84863240806WOS:000301638900002