大三的时候曾经自作聪明地去数学系听了一个学期的拓朴，因为据说拓朴是最没有用的数学。这个闭门造车的学科果然是没有用处，之后就再也没有听说过这个名词，以至刚才拼的时候还犹豫了很久是哪个”朴“……现在居然有聪明人把它用到了国防科技：

Suppose you have a network of sensors, each with a unique ID, scattered around a two-dimensional domain D---for example, D could be a region of forest, an open field, or a portion of the ocean floor. The sensors have a "broadcast radius", within which they can detect the identity of any other sensor, and a "cover radius", within which the sensors perform their sensing tasks. You can think of each sensor as surrounded by a disk whose radius is the coverage radius. The union of these disks is the "sensor cover". A basic question is, Does the sensor cover contain D?