COHERENT RINGS OF FINITE WEAK GLOBAL DIMENSION

The category of left modules over right coherent rings of finite weak global dimension has several nice features. For example, every left mo dule over such a ring has a flat cover (Belshoff, Enochs, Xu) and, if the weak global dimension is at most two, every left module has a flat envelope (Asensio, Martinez). We will exploit these features of this category to study its objects. In particular, we will consider orthogo nal complements (relative to the extension functor) of several classes of modules in this category. In the case of a commutative ring we des cribe an idempotent radical on its category of modules which, when the weak global dimension does not exceed 2, can be used to analyze the s tructure of the flat envelopes and of the ring itself.