Essential Extensions, Excellent Extensions and Finitely Presented Dimension

In Section 1 of this paper, we investigate the finitely presented dimension of an essential extension for a module, and obtain results concerning an essential extension of a torsion-free module. We partially answer the question: When is an essential extension of a finitely presented module (an almost finitely presented module) also finitely presented (almost finitely presented)? In Section 2, we study the C-excellent extensions and the finitely presented dimensions. We obtain some results on the homological dimensions of matrix rings and group rings.