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August 16th, 2012, 11:13 AM   #1
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Let (G,*) is group and A,B are her subgroups.
- Prove that A?B is subgroup of (G,*).
- If |A|=10 and |B| =7 how many elements A?B have and who are they?
- Find subgroups A,B from (Z,+) so that A?B isn't subgroup of (Z,+).
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August 23rd, 2012, 06:45 AM   #2
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Re: Groups

Have you made no attempt at all yourself? It should not be difficult. Since A and B are 'subgroups' of G they are subsets of G and so is their intersection. So the only thing that remains to be shown is that the intersection is 'closed under the operation". If x and y are member so then they are both members of A and so x+ y is in A. Now do the same thing with B.

"If |A|=10 and |B| =7 how many elements A?B have"

is closed under the operation and so is a subgroup of B. The crucial point is that |B|= 7 is a prime number. What does that tell you about the subgroups of B?

"Find subgroups A,B from (Z,+) so that A?B isn't subgroup of (Z,+)."
I'll give you a hint. All subgroups of (Z,+) are of the form {in| is is any integer, n is a fixed integer}. In other words, they are "all even numbers", "all multiples of three", etc.
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