The classes of clean and nil-clean rings are closed with respect standard constructions as direct products and (triangular) matrix rings, cf. [12] resp. [4], while the classes of weakly (nil-)clean rings are not closed under these constructions. Moreover, while all matrix rings over fields are clean, [12] when we consider nil-clean rings there are strongly restrictions: if a matrix ring over a ...

Preparation • Dissolve 1 heaped teaspoonful of salt or sodium bicarbonate in a jug containing 500 ml of boiled water (half a litre); allow this solution to cool (Figure 1). • Pour a very small amount of the solution into a small sterile pot on a clean surface (Figure 2). Method 1 The eyelashes • Ask the patient to close both eyes. • Take a folded gauze swab or cotton bud. • Moisten the swab or ...

We develop a microeconomic model of endogenous growth where clean and dirty technologies compete in production and innovation– in the sense that research can be directed to either clean or dirty technologies. If dirty technologies are more advanced to start with, the potential transition to clean technology can be diffi cult both because clean research must climb several rungs to catch up with ...