Is the Relation R on Groups an Equivalence Relation?

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Homework Help Overview

The discussion revolves around the relation R defined on the set of all groups, specifically whether this relation, where H is a subgroup of K, qualifies as an equivalence relation. Participants explore the properties that define equivalence relations, including reflexivity, symmetry, and transitivity.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants examine the definition of R and its implications for equivalence relations, questioning the validity of the properties of reflexivity, symmetry, and transitivity in this context. Some express confusion over the phrasing of the original statement and seek clarification on the properties of equivalence relations as they apply to groups.

Discussion Status

The discussion is active, with participants articulating their understanding of the properties of equivalence relations and how they relate to the defined relation R. There is acknowledgment of the need to verify each property, and some participants have begun to outline their reasoning regarding reflexivity and transitivity, while symmetry remains a point of contention.

Contextual Notes

Participants note that the original statement may be ambiguous, leading to different interpretations of the relation R and its classification as an equivalence relation. There is also a recognition that the properties must be checked rigorously to determine the nature of R.

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Homework Statement


The relation R on the set of all groups defined by HRK if and only if H is a subgroup of K is an equivalence relation.


Homework Equations


Subgroup: has identity, closed under * binary relation, has inverse for each element.
Equivalence relation: transitive, symmetric, and reflexive.


The Attempt at a Solution


I know that the answer is false, but I'm having trouble parsing the question. Any help would be greatly appreciated!
 
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The statement is about the equivalence relation. So if you want to prove that R is or is not an equivalence relation, you will need to check the three properties of an equivalence relation.

Can you express more concretely what the three properties say in this case (say, if H, J, K are arbitrary groups).
 
CompuChip said:
The statement is about the equivalence relation. So if you want to prove that R is or is not an equivalence relation, you will need to check the three properties of an equivalence relation.

Can you express more concretely what the three properties say in this case (say, if H, J, K are arbitrary groups).

So is this still the same question? (I reworded it a bit to make it more understandable for myself): The relation R is an equivalence relation on the set of all groups defined by HRK if and only if H is a subgroup of K.

So R is not necessarily an equivalence relation because 3 conditions were not satisfied:
HRH for all groups, so H is a subgroup of itself, which is true, so reflexive is satisfied.
If HRK, then KRH. If H is a subgroup of K, then K is a subgroup of H. This is false, so symmetry does not follow.
Assume HRK and KRJ, then HRJ. If H is a subgroup of K, and K is a subgroup of J, then H is a subgroup of J. This is true, so transitivity is satisfied.
 
Exactly.
Note that the "if and only if" part is in the definition of R, it does not apply to it being an equivalence relation. That means:
R is defined by the following statement: HRK is true if and only if H is a subgroup of K​
which is something else than
R is an equivalence relation, if and only if it is true that H is a subgroup of K​
which is clearly nonsense (you have not even said what H and K are there).
 

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