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Mr Davis 97
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Take the subset of ##\mathbb{R}##, ##X = [0,1]\cup [2,3]##. Under the usual metric, the set ##X## is open and closed, according to my text. How is this the case? In particular, how is ##[0,1]## open in ##X##?
##[0,1] =(-0.1,1.1) \cap [0,1] \cup [2,3]##. This satisfies the definition of open in subspaces.Mr Davis 97 said:Take the subset of ##\mathbb{R}##, ##X = [0,1]\cup [2,3]##. Under the usual metric, the set ##X## is open and closed, according to my text. How is this the case? In particular, how is ##[0,1]## open in ##X##?
See https://www.physicsforums.com/threa...-of-continuous-functions.960345/#post-6091723 for a bit more detailed explanation.Mr Davis 97 said:Take the subset of ##\mathbb{R}##, ##X = [0,1]\cup [2,3]##. Under the usual metric, the set ##X## is open and closed, according to my text. How is this the case? In particular, how is ##[0,1]## open in ##X##?
I suppose I understand now how to show that something IS open or closed, but how do you show that something is not? For example, why isn't ##(1/4,3/4)## closed in ##(1/2,1]##? Do I have to show that for all closed subsets ##C## of ##\mathbb{R}##, that ##(1/4,3/4) \not = (1/2,1] \cap C##?WWGD said:##[0,1] =(-0.1,1.1) \cap [0,1] \cup [2,3]##. This satisfies the definition of open in subspaces.
You do it as usual. Show that there is a sequence in ##(1/4,3/4)## which converges to, say ##3/4\,.##Mr Davis 97 said:I suppose I understand now how to show that something IS open or closed, but how do you show that something is not? For example, why isn't ##(1/4,3/4)## closed in ##(1/2,1]##? Do I have to show that for all closed subsets ##C## of ##\mathbb{R}##, that ##(1/4,3/4) \not = (1/2,1] \cap C##?
Easiest way (I think): [2,3] is closed in ##X##. Since [itex] [0, 1] = X -[2, 3][/itex], [0,1] is the complement in ##X## of [2,3] and so must be open.Mr Davis 97 said:Take the subset of ##\mathbb{R}##, ##X = [0,1]\cup [2,3]##. Under the usual metric, the set ##X## is open and closed, according to my text. How is this the case? In particular, how is ##[0,1]## open in ##X##?
A set being both open and closed means that it satisfies the conditions of being open and closed at the same time. In other words, the set contains all of its boundary points and also has no boundary points.
This concept can be a bit confusing, but it is possible for a set to satisfy both the conditions of being open and closed. This usually occurs in sets with discrete elements, where all points are both interior and exterior points of the set.
A set being both open and closed has important implications in topology and real analysis. It allows for the existence of discontinuous functions and helps to define the concept of connectedness in a topological space.
One example of a set that is both open and closed is the set of all integers. This set contains all of its boundary points (the integers themselves) and has no boundary points since it is discrete. Therefore, it satisfies both conditions of being open and closed.
A set being both open and closed can be seen as a special case of a closed set, where the set is also open. In other words, the closure of a set is the smallest closed set that contains the original set, and in some cases, this smallest closed set can also be open.