拓扑是研究几何图形或空间在连续改变形状后还能保持不变的一些性质的一个学科。 它只考虑物体间的位置关系而不考虑它们的形状和大小。 拓扑英文名是Topology，直译是地志学，最早指研究地形、地貌相类似的有关学科。 几何拓扑学是十九世纪形成的一门数学分支，它属于几何学的范畴。
Topology is the mathematical study of the properties that are preserved through deformations, twistings, and stretchings of objects. Tearing, however, is not allowed. A circle is topologically equivalent to an ellipse (into which it can be deformed by stretching) and a sphere is equivalent to an ellipsoid. Similarly, the set of all possible positions of the hour hand of a clock is topologically equivalent to a circle (i.e., a one-dimensional closed curve with no intersections that can be embedded in two-dimensional space), the set of all possible positions of the hour and minute hands taken together is topologically equivalent to the surface of a torus (i.e., a two-dimensional a surface that can be embedded in three-dimensional space), and the set of all possible positions of the hour, minute, and second hands taken together are topologically equivalent to a three-dimensional object.
The definition of topology leads to the following mathematical joke (Renteln and Dundes 2005):
Q: What is a topologist? A: Someone who cannot distinguish between a doughnut and a coffee cup.
There is more to topology, though. Topology began with the study of curves, surfaces, and other objects in the plane and three-space. One of the central ideas in topology is that spatial objects like circles and spheres can be treated as objects in their own right, and knowledge of objects is independent of how they are “represented” or “embedded” in space. For example, the statement “if you remove a point from a circle, you get a line segment” applies just as well to the circle as to an ellipse, and even to tangled or knotted circles, since the statement involves only topological properties.
Topology has to do with the study of spatial objects such as curves, surfaces, the space we call our universe, the space-time of general relativity, fractals, knots, manifolds (which are objects with some of the same basic spatial properties as our universe), phase spaces that are encountered in physics (such as the space of hand-positions of a clock), symmetry groups like the collection of ways of rotating a top, etc.
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