Let d be the plane area. If D is connected by roads, and the part surrounded by any closed curve in D belongs to D, it is called a plane simply connected region. Note that some textbooks (such as Mathematical Analysis, 3rd Edition, Volume 2, edited by Ouyang Guangzhong, Zhu Xueyan, Jin Fulin, Chen, etc.) don't mention "D must be connected by roads" when defining the plane simple connected area, which is actually wrong. More generally, a simply connected region is a region without a "hole".