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bxn4
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I am struggling to prove the following: Let E be a compact nonempty subset of R^k and let delta = {d(x,y): x,y in E}. Show E contains points x_0,y_0 such that d(x_0,y_0)=delta.
bxn4 said:I am struggling to prove the following: Let E be a compact nonempty subset of R^k and let delta = {d(x,y): x,y in E}. Show E contains points x_0,y_0 such that d(x_0,y_0)=delta.
jostpuur said:Am I correct to guess that you meant the supremum
[tex]
\Delta := \sup\{d(x,y)\;|\;x,y\in E\}\; ?
[/tex]
It is convenient to consider a function [itex]d:E\times E\to\mathbb{R}[/itex], and use some basic topological results, or their immediate consequences. For example: The Cartesian product of compact sets is a compact set. In metric spaces compact sets are sequentially compact. The distance function d is continuous. Continuous mappings map compact sets into compact sets. The Heine-Borel Theorem. Just put pieces together!
A compact subset of R^n is a subset of n-dimensional Euclidean space that is closed and bounded. This means that the subset contains all of its boundary points and is contained within a finite region of space.
A subset of R^n is compact if it is both closed and bounded. To determine if a subset is closed, you can check if all of its limit points are contained within the subset. To check if a subset is bounded, you can see if it is contained within a finite region of space.
Examples of compact subsets of R^n include a closed interval in one dimension, a closed disk in two dimensions, and a closed ball in three dimensions. In general, any closed and bounded subset of n-dimensional Euclidean space is considered compact.
Compact subsets are important in mathematics because they have nice topological properties that make them easier to study and analyze. They also allow for the application of powerful theorems, such as the Heine-Borel theorem and the Bolzano-Weierstrass theorem.
In general, a function is continuous on a compact subset if and only if it is uniformly continuous. This means that the function's behavior on the subset is consistent and predictable. Additionally, compact subsets are useful for analyzing convergence of sequences and series, as they provide a framework for understanding when and how a sequence or series will converge.