Proving that an equivalence relation is a bijection

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Homework Help Overview

The problem involves an equivalence relation defined on pairs of real numbers, where two pairs are considered equivalent if the sum of the squares of their components is equal. The task is to prove that there exists a bijection between the set of all positive real numbers (including zero) and the set of equivalence classes formed by this relation.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the nature of the equivalence relation and its geometric interpretation, questioning whether it relates to points being on the same circle. There are attempts to define a mapping from the equivalence classes to the positive reals based on the radius squared, but uncertainty remains about establishing a bijection.

Discussion Status

Some participants have suggested a mapping that relates the equivalence classes to their common radius, indicating a potential direction for proving the bijection. However, there is a lack of clarity on how to formally prove that this mapping is a bijection, particularly in the context of equivalence classes.

Contextual Notes

Participants express concern about the lack of examples in their resources regarding proving bijections in the context of equivalence classes, which adds to the difficulty of the problem as a final exam approaches.

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Homework Statement



Let (a, b), (c, d) be in R x R. We define (a, b) ~ (c, d) iff a^2 + b^2 = c^2 + d^2.

Let R* = all positive real numbers (including 0).

Prove that there is a bijection between R* and the set of all equivalence classes for this equivalence relationship.

Homework Equations


The Attempt at a Solution



I do not know how the formula for the relationship ~ looks like. I tried mapping from R* x R* -> R* where f: x^2 + y^2 but I got stuck. Any help please?
 
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number0 said:

Homework Statement



Let (a, b), (c, d) be in R x R. We define (a, b) ~ (c, d) iff a^2 + b^2 = c^2 + d^2.

Let R* = all positive real numbers (including 0).

Prove that there is a bijection between R* and the set of all equivalence classes for this equivalence relationship.

Homework Equations




The Attempt at a Solution



I do not know how the formula for the relationship ~ looks like. I tried mapping from R* x R* -> R* where f: x^2 + y^2 but I got stuck. Any help please?

Doesn't (a, b) ~ (c, d) mean the two points are on the same circle centered at the origin? Is that what you mean by "what it looks like"?
 
LCKurtz said:
Doesn't (a, b) ~ (c, d) mean the two points are on the same circle centered at the origin? Is that what you mean by "what it looks like"?

I knew what it looks like... but I do not know the equation is supposed to be set up. For example, I did f(x,y) = x^2 + y^2 but I could not find injection.
 
Last edited:
number0 said:
I knew what it looks like... but I do not know the equation is supposed to be set up. For example, I did f(x,y) = x^2 + y^2 but I could not find injection.

Well, it's true that the radius is common to all the elements in an equivalence class. So I would say you are on the right track. Try the map that maps the equivalence class for (a,b) into the common radius or radius squared. If you denote the equivalence class for (a,b) by
[(a,b)] your map could map that to a2+b2. Then you have to show your function is well defined and has all the necessary properties to be a bijection.
 
LCKurtz said:
Well, it's true that the radius is common to all the elements in an equivalence class. So I would say you are on the right track. Try the map that maps the equivalence class for (a,b) into the common radius or radius squared. If you denote the equivalence class for (a,b) by
[(a,b)] your map could map that to a2+b2. Then you have to show your function is well defined and has all the necessary properties to be a bijection.

Okay, this may sound really bad... but I do not know how to prove bijection in equivalence classes (I know how to do it for functions but my book never showed me examples about proving bijection in equivalence classes). I tried googling examples but I cannot find any. My finals is tomorrow! Can anyone help me?
 

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