An important topic of rough set theory is the approximations of undefined concept through defined concept,and its basic work is how to construct the defined concept and approximation operators.

• 粗糙集理论的一个重要研究方面是用已定义的概念来近似未定义的概念,而如何构建可定义概念以及如何确定近似运算是这一工作的基础。