Constructive bijection between [0,1] and R?

  • Thread starter nomadreid
  • Start date
  • Tags
    Bijection
In summary, it is possible to construct a bijection between [0,1] and ℝ by defining g and h as described in the conversation. This bijection is not continuous, but it is still valid and elegant.
  • #1
nomadreid
Gold Member
1,672
206
It is easy to construct a bijection between the open interval (0,1) and ℝ, and (if one isn't an intuitionist) it is easy to prove that there exists a bijection between [0,1] and ℝ, but is it possible to construct such a bijection between [0,1] and ℝ? Obviously it won't be continuous, but that's OK.
 
Physics news on Phys.org
  • #2
Define [itex]g: \mathbb{R} \to \mathbb{R}[/itex] by [tex]
g : x \mapsto \begin{cases} x, & x \neq 0, 1, 2, 3, \dots, \\
x + 2, & x = 0, 1, 2, 3, \dots .\end{cases}[/tex] Note that [itex]g[/itex] is an injection and its image is [itex]\mathbb{R} \setminus \{0,1\}[/itex].

Now take any bijection [itex]f: (0,1) \to \mathbb{R}[/itex], and define [itex]h: [0,1] \to \mathbb{R}[/itex] by [tex]
h: x \mapsto \begin{cases} (g \circ f)(x), & x \in (0,1), \\
0, & x = 0, \\
1, & x = 1.\end{cases}[/tex] Then [itex]h[/itex] is a bijection.
 
  • Like
Likes jim mcnamara, nomadreid and FactChecker
  • #3
Ooooh, that's elegant. I like it! Thanks very much, pasmith. :smile:
 

Related to Constructive bijection between [0,1] and R?

1. What is a constructive bijection between [0,1] and R?

A constructive bijection is a function that maps each element in the interval [0,1] to a unique element in the set of real numbers (R), and vice versa. This means that every number in [0,1] has a corresponding number in R, and every number in R has a corresponding number in [0,1].

2. Why is a constructive bijection between [0,1] and R important?

A constructive bijection allows us to establish a one-to-one correspondence between the interval [0,1] and the set of real numbers. This is important in mathematics and computer science, as it allows us to compare and analyze different sets of numbers, and also to map data from one domain to another.

3. How is a constructive bijection between [0,1] and R different from other types of bijections?

A constructive bijection is a type of bijection where the mapping between [0,1] and R can be explicitly defined and constructed. This means that we can explicitly define the function and its inverse, rather than just proving that one exists. Other types of bijections may not have this constructibility property.

4. What is the formula for a constructive bijection between [0,1] and R?

There are many possible formulas for a constructive bijection between [0,1] and R, as long as the function is continuous, one-to-one, and onto. One commonly used formula is f(x) = tan((x-1/2)π), which maps the interval [0,1] to the set of real numbers between -∞ and ∞. However, there are many other possible formulas that can be used.

5. Can a constructive bijection between [0,1] and R be extended to other intervals?

Yes, a constructive bijection between [0,1] and R can be extended to other intervals, as long as the interval is a subset of [0,1] or R. For example, we can extend the bijection to the interval [0,2] by simply doubling the range of values for each input, resulting in the function f(x) = 2tan((x-1)π/2). However, it is not possible to extend a constructive bijection from [0,1] to a larger interval, such as [0,3], while maintaining all the properties of a bijection.

Similar threads

Replies
15
Views
2K
  • Set Theory, Logic, Probability, Statistics
Replies
18
Views
2K
  • Calculus and Beyond Homework Help
Replies
1
Views
540
  • Set Theory, Logic, Probability, Statistics
2
Replies
55
Views
5K
  • Set Theory, Logic, Probability, Statistics
Replies
26
Views
3K
  • Differential Equations
Replies
1
Views
719
  • Calculus and Beyond Homework Help
Replies
5
Views
2K
  • General Math
Replies
10
Views
3K
  • Calculus and Beyond Homework Help
Replies
4
Views
1K
  • Set Theory, Logic, Probability, Statistics
Replies
2
Views
4K
Back
Top