翻译：密度直达：若xi位于xj的 邻域中且xi是核心对象则称xi由xj密度直达。但是反之不一定正确即此时并不能确定xj由xi密度直达 除非且xi也是核心对象。

Density Reachability: If xi is located in the neighborhood of xj and xi is a core object, then xi is said to be density-reachable from xj. However, the reverse may not be true, that is, it cannot be determined that xj is density-reachable from xi unless xi is also a core object.