Is H a Subgroup if xy Belongs to H for All x, y in H, and Why Isn't Q Cyclic?

In summary, we discussed two problems related to group theory. The first problem involved proving that a finite subset H of a group G is a subgroup if and only if xy belongs to H whenever x and y belong to H. We explored both directions of this proof, using the fact that G is a group and applying the concept of closure over multiplication. The second problem involved showing that the group of rational numbers (Q) with addition is not cyclic. We used a proof by contradiction, showing that if Q were cyclic, there would be an element x such that <x>=Q, but x/2 would not be in <x>, leading to a contradiction.
  • #1
jacobrhcp
169
0
[SOLVED] group theorem problems

Homework Statement



1) suppose H is a finite subset of a group G. Prove that H is a subgroup of G if and only if xy belongs to H whenever x and y belong to H
2) show that Q is not cyclic, (where Q is the group of rationals with addition.)

The Attempt at a Solution



1) if H is a subgroup, then if x,y are in H, so is by hypothesis xy, because H is a subgroup and therefore closed.

Now conversely, if xy is an element of H when x,y are in H, then x*x is in H
Let n be the number of elements in H, then x^n is in H
Now because H is finite, there has to be an element such that x^n+1=x^m, with n+1>m. Otherwise there would be n+1 distinct elements in a set with n elements. I didn't get any further then this.

I suppose if for elements of H a*b=a*c would imply that b=c, then x=x^n+1-m and so e:=x^n-m. Then I can find and inverse too without many problems, and because H is already know to have an associative operation, and H is closed by hypothesis, H is a subgroup. But is this true?

2) Suppose there is an element x such that <x>=Q. Then x/2 is too in Q, and is not in <x>. But I don't find this a very convincing proof by contradiction.
 
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  • #2
1) Are you sure you aren't missing an inverse sign somewhere? As in: H is a subgroup of G iff for all x and y in H, [itex]x y^{-1} \in H[/itex] ? There was a case where you could do without the inverses (G abelian?), but in general you need it.

2) Are Q the rationals, or the rationals - 0 or the quaternions or ...? A group consists of a set and an operation, what is the operation (addition, multiplication, ...).
 
  • #3
In part 1, you can get a lot of mileage out of the fact that [itex]G[/itex] is a group, so every element of [itex]H[/itex] has some order in [itex]G[/itex].

Compuchip: Since [itex]H[/itex] is finite, closure over multiplication is sufficient.
 
  • #4
compu: for 2) H is finite, which makes the difference. And Q are the rationals (with addition).

nate, what exactly do you mean by 'all elements having some order in G', and what is it's use?
 
  • #5
jacobrhcp said:
compu: for 2) H is finite, which makes the difference. And Q are the rationals (with addition).
You're right, I seem to be quite rusty in this subject.

jacobrhcp said:
nate, what exactly do you mean by 'all elements having some order in G', and what is it's use?
He means that for every element x in G, there is some number n such that x^n = e (not necessarily the same n for all x though).

The only thing you know is that H is non-empty, so there must be at least one element, x. Now use what nate told you. You will be able to fulfill at least two of the group axioms for H in one blow.
 
  • #6
CompuChip said:
...
He means that for every element x in G, there is some number n such that x^n = e (not necessarily the same n for all x though)...

Actually, [itex]G[/itex] can admit elements of infinite order since it is not necessarily finite. (Of course, those elements can't be part of [itex]H[/itex].)
 
  • #7
for 1) take any x in H, then x, x^2, x^3, ... are all elements of H, but H is finite, so at least two of these must be the same, say x^j = x^k for some j != k. You should be able to take it from here.
 
  • #8
ircdan, the problem with that argument is that you are not sure that xa=xb implies that a=b. But I think I got it now, thanks everyone!

Anyone got any tips to show Q is not cyclic?
 
  • #9
jacobrhcp said:
2) Suppose there is an element x such that <x>=Q. Then x/2 is too in Q, and is not in <x>. But I don't find this a very convincing proof by contradiction.

It's almost ok! You need to rule out 0 at the beginning and this is obvious since <0> != Q. So suppose Q was cyclic, then there is x !=0 in Q s.t. <x> = Q. Then x/2 is in Q, but x/2 is not in <x>.
This is enough, because <x> = {nx | n in Z} and x/2 is not of the form n*x for n in Z.

Now if you are not convinced, suppose that x/2 was in <x>, then x/2 = nx for some n in Z.

Then, x= (2n)x, so (2n-1)x = 0(ie, this means x + x + ... + x = 0 (2n-1) times), so x has finite order, but only 0 has finite order in Q, so x = 0, a contradiction.
 
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  • #10
jacobrhcp said:
ircdan, the problem with that argument is that you are not sure that xa=xb implies that a=b. But I think I got it now, thanks everyone!

Anyone got any tips to show Q is not cyclic?

There is no problem there. If xa = xb in H, then this is an equation in G, so a = b. In our case everything is a power of x and x is in H, so everything is in H by hypothesis, so our inverse lies in H. It's a good trick to keep in mind!
 
  • #11
thanks, and solved!
 

FAQ: Is H a Subgroup if xy Belongs to H for All x, y in H, and Why Isn't Q Cyclic?

What is a group theorem problem?

A group theorem problem involves using mathematical concepts and principles known as group theory to solve complex problems related to group structures and operations. Group theory is a branch of abstract algebra that studies the properties of groups, which are mathematical structures consisting of a set of elements and an operation that combines any two elements to produce a third.

What are the key components of a group theorem problem?

The key components of a group theorem problem include the set of elements, the operation that combines elements, and the properties of the group. These properties include closure (the result of an operation on two elements is also an element of the group), associativity (the order of operations does not matter), identity (there is an element that produces the same result when combined with any other element), and invertibility (every element has an inverse that, when combined, produces the identity element).

How are group theorem problems solved?

Group theorem problems are solved by applying the principles and properties of group theory to the given problem. This involves identifying the group structure and operation, determining the properties of the group, and using mathematical techniques such as permutation and combination, Lagrange's theorem, and Cayley's theorem to solve the problem.

What are some real-world applications of group theorem problems?

Group theorem problems have many real-world applications, including cryptography, chemistry, physics, and computer science. In cryptography, group theory is used to develop secure encryption algorithms. In chemistry, it is used to understand the symmetries of molecules. In physics, it is used to study the symmetries of physical systems. In computer science, it is used to design efficient algorithms and data structures.

What are some tips for solving group theorem problems?

Some tips for solving group theorem problems include understanding the properties of groups, practicing with different types of problems, breaking down complex problems into smaller, more manageable parts, and utilizing mathematical techniques and theorems specific to group theory. It is also helpful to draw diagrams or use visual aids to better understand the group structure and operation.

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