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It is just an open set because it is just a part of open covering.fresh_42 said:How are the ##H_i## defined?
Ok. Now, if you take ##ε = 2(β-α)##, how many ##ε##-intervals around a fixed ##x_0## would you need for coverage? How many if ##ε = \frac{β-α}{2}##, how many for ##ε = \frac{β-α}{3}## or ##ε = \frac{β-α}{n}## with a fixed ##n##? The limit is essentially the union of all points of ##[α,β]## but for a given open coverage, finitely many will be enough.kidsasd987 said:It is just an open set because it is just a part of open covering.
fresh_42 said:Ok. Now, if you take ##ε = 2(β-α)##, how many ##ε##-intervals around a fixed ##x_0## would you need for coverage? How many if ##ε = \frac{β-α}{2}##, how many for ##ε = \frac{β-α}{3}## or ##ε = \frac{β-α}{n}## with a fixed ##n##? The limit is essentially the union of all points of ##[α,β]## but for a given open coverage, finitely many will be enough.
The point is: For any given open coverage of a compact set (in a metric space - although I think Hausdorff is enough) there is a finite sub-coverage. Again: a given coverage!kidsasd987 said:Thanks for your reply.
I wanted to confirm the theorem, "if a set is compact then its open covering will contain a subcovering that consists of finitely many open sets." I believe H is an open covering, since it contains S and consists of open sets. However, because epsilon is converging to 0, it would require infinitely many sets to fully cover the set S, which contradicts to the theorem.
Each open set overlaps as much as epsilon, but it is shrinking to 0. Then would we need finitely many open sets Hi to cover the closed set? Somehow I feel there has to be infinitely many.
Hmm, can I understand this way.fresh_42 said:The point is: For any given open coverage of a compact set (in a metric space - although I think Hausdorff is enough) there is a finite sub-coverage. Again: a given coverage!
If you take ##ε=0## then you simply have ##S= \{x | x \in S\} = \bigcup_{x \in S} \{x\}##. This is no open coverage anymore! But as soon as you chose a ##ε > 0##, how small it ever maybe, then finitely many will be sufficient.
I don't see why you need a ##ε'##, but yes, this is correct. Perhaps you shouldn't say countable because this usually refers to an infinite number of things like the amount of natural numbers, since a finite number of things are always countably many.kidsasd987 said:Hmm, can I understand this way.εεε
no matter how small ε is, we can find ε'< ε. which means ε is bounded below by some ε'. Then we can find an integer n which satisfies 1/n < ε.
Also 1/n'<ε'.
since 1/n is bounded by 1/n' and countable therefore finite number of open sets will be enough?
fresh_42 said:I don't see why you need a ##ε'##, but yes, this is correct. Perhaps you shouldn't say countable because this usually refers to an infinite number of things like the amount of natural numbers, since a finite number of things are always countably many.
Compactness refers to the quality of being small, tightly packed, or dense. In mathematics and physics, it is a concept used to describe the size or shape of an object or space.
In mathematics, compactness is measured using a concept called "compactness" or "compactness index". This index is a numerical value that represents how much an object can be shrunk without losing any of its essential properties. In physics, compactness can be measured by calculating the mass-to-size ratio of an object.
Compactness is an important concept in science because it allows us to understand and describe the physical properties of objects and spaces. In mathematics, it helps us to analyze and solve problems involving geometric shapes and sets. In physics, it helps us to understand the behavior of matter and energy in the universe.
Compactness and density are closely related concepts. While compactness refers to the tightness or density of an object or space, density specifically measures the amount of mass within a given volume. In other words, density is a measure of how compact an object is. Objects with high compactness also tend to have high density.
Yes, compactness can be changed through various processes. In physics, objects can be compressed or expanded, which changes their compactness. In mathematics, shapes and sets can be transformed, which can also alter their compactness. In addition, compactness can also be affected by external factors such as pressure, temperature, and gravity.