 In this paper, for the first time, a comprehensive literature review is provided to summarize the ten-year development in DG . First, we cover the background by giving the problem definitions and discussing how DG is related to other fields like domain adaptation and transfer learning.
<clarity>  In this paper, for the first time, a comprehensive literature review is provided to summarize the ten-year development in DG . First, we cover the background by giving the problem definitions and discussing how DG is related to other fields like domain adaptation and transfer learning.
 In this paper, for the first time, a comprehensive literature review is provided to summarize the ten-year development in DG . First, we cover the background by formally defining DG and relating it to other research fields like domain adaptation and transfer learning.