Chapter 6: Problem 11
Give an example of a convex set whose complement is bounded.
Short Answer
Expert verified
An example is the entire space \( \mathbb{R}^n \), whose complement is the empty set, which is bounded.
Step by step solution
01
Understand Definitions
To solve the problem, we must first understand what a convex set is. A set is convex if, for any two points within the set, the line segment connecting them lies within the set as well. Additionally, a set is bounded if it fits within a sufficiently large ball centered at origin.
02
Identify Complement and Boundedness
The problem asks for a convex set whose complement is bounded. This means we are looking for a set where everything outside this set (the complement) is limited in extent, i.e., does not extend to infinity.
03
Example - The Empty Set
Consider the empty set, denoted by \( \emptyset \). The empty set is technically a convex set because there are no points inside it, so no counterexamples to convexity exist. Its complement is the entire space (say, \( \mathbb{R}^n \)), which is not bounded.
04
Correct Answer - Full Space
Based on the problem constraints, consider using the full space \( \mathbb{R}^n \). Its complement would be the empty set, which *is* bounded because it contains no points, satisfying the condition that its 'complement is bounded'.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Bounded Set
A set is known as a "bounded set" if it can be completely contained within a finite region of space. In simpler terms, if you can imagine drawing a large circle (or using any geometric shape) around the entire set, without ever needing to make the circle infinitely large, then the set is bounded.
To illustrate this:
- Think of a small group of dots drawn on a piece of paper. You can easily draw a circle around them, which means the set of these dots is bounded.
- In contrast, if the dots continue infinitely in all directions, you would never be able to fully encompass them with a single circle, so the set would not be bounded.
Complement of a Set
The "complement of a set" consists of all the elements that are not in the original set, within a universal context. To better understand this, imagine a big universe of elements where a smaller subset is removed. The remaining elements constitute the complement of the original subset.For example:
- If you have a set containing all vowels from the alphabet, then the complement would be all the consonants, assuming the universal set is the entire English alphabet.
- More formally, if set A denotes a subset of the universal set U, the complement of A (\( A^c \)) is defined as \( A^c = \{ x \in U : x otin A \} \).
Empty Set
The "empty set" is a fundamental concept in mathematics. It is the set that contains no elements at all and is often represented by the symbol \(\emptyset\). Here are a few key points:
- It is unique, meaning there is only one empty set, as it doesn't contain anything that can differentiate it from another empty set.
- The empty set is considered a subset of every set, which means if you take any set, the empty set is always one of its subsets.
- Mathematically, it has special properties such as being both an open and closed set, as well as being convex, since there are no points to challenge this property.