Equivalence Relation: Can't Understand x~x+1 on Real Numbers

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    Equivalence Relation
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Discussion Overview

The discussion revolves around the equivalence relation defined by x ~ x+1 on the real numbers, particularly focusing on its properties and implications in the context of quotient spaces. Participants explore whether this relation qualifies as an equivalence relation and seek clarification on its meaning and application.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about how x ~ x+1 can be considered an equivalence relation on the real numbers.
  • Another suggests that the relation might be interpreted as generating a sequence (x ~ x+1 ~ x+2 ~ ...), indicating a lack of context for understanding.
  • A participant outlines the properties of equivalence relations—reflexivity, symmetry, and transitivity—and argues that the relation fails to meet these criteria.
  • One participant speculates that the relation could relate to the real numbers mod 1.
  • After clarification of context, another participant agrees that the relation corresponds to the quotient space of real numbers modulo 1, asserting that it is indeed an equivalence relation as stated in literature.
  • A later reply summarizes that the equivalence classes under this relation include all real numbers differing by integers, such as Z+{0.5}.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the initial understanding of the relation as an equivalence relation. While some argue it does not satisfy the properties of equivalence relations, others assert that it does when considered in the context of quotient spaces.

Contextual Notes

There is a lack of clarity regarding the initial interpretation of the relation, and assumptions about its context influence the discussion. The properties of equivalence relations are debated, particularly in relation to the specific case of real numbers modulo 1.

andlook
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Hey

I can't see how x~x+1 is an equivalence relation on the real numbers?

I don't understand what the relation is. Can anyone help?
 
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Perhaps you want the equivalence relation generated by this...

x ~ x+1 ~ x+2 ~ ...

Or perhaps not. Without context, we can only guess.
 
A relation is a rule that relates elements of one set (in this case the real numbers) to elements of another set (in this case, ALSO the real numbers). An equivalence relation is one from a set to itself that has three properties:

The relation must be reflexive (x must be related to x)
The relation must be symmetric (if x is related to y, then y is related to x), and
The relation must be transitive (if x is related to y, and y is related to z, then x is related to z)

The relation you have, where x is related to x+1 is not an equivalence relation.
1 is not related to 1, since 1 is only related to 2 (not reflexive).
1 is related to 2, but 2 is not related to 1 (it's related to 3) (not symmetric).
1 is related to 2 and 2 is related to 3, but 1 is not related to 3 (not transitive).

It actually fails every single one of the properties.
 
g_edgar said:
Without context, we can only guess.

My guess: it's the real numbers mod 1.
 
Oops yeah should have been a lot more specific. Context:

Talking about the quotient space of r by the equivalence relation x ~ x+1.

Relating each point to the point +1 ?

1~2 and 2~3 but 1~3 is false...

This is an equivalence relation since states so in literature. So it is clear I don't understand how equivalence relations are working here. Any help? Thanks
 
Ah, then CRGreathouse was right; the quotient space on that relation is constructing the real numbers modulo 1. When it's flat out saying that that's an equivalence relation, then it's saying that it's reflexive, transitive and symmetric. Essentially, this is saying that the part of any real number before the decimal point doesn't matter, so the equivalence classes (all the things that are equivalent under this relation) are things like Z+{0.5} = {..., -3.5, -2.5, -1.5, -0.5, 0.5, 1.5, 2.5, 3.5, ...}, because all those things are related to each other.
 
ok so R / ~ = [0,1)?
 

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