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Mathematics LibreTexts

4: Topology of the Real Line

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    22663
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    Thumbnail: The blue circle represents the set of points (x, y) satisfying \(x^2 + y^2 = r^2\). The red disk represents the set of points (x, y) satisfying \(x^2 + y^2 < r^2\). The red set is an open set, the blue set is its boundary set, and the union of the red and blue sets is a closed set. (Public Domain; Richard Giuly via Wikipedia)

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