| 摘要 | 第4-5页 |
| Abstract | 第5-6页 |
| Chapter 1 Preface | 第8-13页 |
| 1.1 Background and main results | 第8-12页 |
| 1.2 Symbols | 第12-13页 |
| Chapter 2 Basic concepts | 第13-32页 |
| 2.1 Set theory | 第13-23页 |
| 2.2 Direct limits, purity and covers | 第23-32页 |
| Chapter 3 Coherent rings and absolutely pure covers | 第32-47页 |
| 3.1 Introduction | 第32-33页 |
| 3.2 Preliminaries | 第33-36页 |
| 3.3 Coherent rings and absolutely pure covers | 第36-43页 |
| 3.4 Cotorsion pairs of absolutely pure modules | 第43-47页 |
| Chapter 4 Is any covering class closed under direct limits? | 第47-60页 |
| 4.1 Introduction | 第47页 |
| 4.2 Two lemmas on tree modules | 第47-51页 |
| 4.3 An equivalent condition on Enochs' question | 第51-60页 |
| REFERENCES | 第60-66页 |
| Papers published during my PhD program | 第66-67页 |
| 致谢 | 第67-68页 |
