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发帖数: 119 | 1 能否举出一个反例，基于一个集合的sigma 代数不是一个拓扑空间？
多谢！ | s**e
发帖数: 15 | 2 你看这个是不是：let X be the set of all real numbers. Y={A|A is a subset of
X, A or X\A is countable}. (X,Y) is not a topological space. Suppose it is a
topological space, then any one point subset is open. It follows that
interval [0,1] is open, which is not in Y. \sigma algebra 要求可数并， 拓扑
空间要求任意并，这大概是二者的本质差别。 |
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