Not so: the union of two subgroups is almost never a subgroup (exercise: when exactly?).Quote:

Originally Posted byThePerfectHacker

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- January 22nd 2006, 02:48 AMrgepQuote:

Originally Posted by**ThePerfectHacker**

- January 22nd 2006, 03:50 AMCaptainBlackQuote:

Originally Posted by**ThePerfectHacker**

and "intersection of two subgroups" to mean. As it is they don't mean

anything as union and intersection are usually defined on sets. And a

groups is not a set, a set may be involved but the group is not the set,

and as rgep points out the naive union/intersection of the associated sets

of a pair of subgroups is not usually the set associated with another subgroup.

RonL - January 22nd 2006, 10:12 AMThePerfectHacker
Maybe I did make a mistake. I just remember my book on Abstract Algebra asking to prove the intersection/uninon of groups, is a group something like that. Maybe, it is true for

**Normal Subgroups**I do not remember exactly.

I do not think I made a mistake. By uninon/intersection I mean respectively. I just am visualizing that in my head, I think it is a subgroup. - January 22nd 2006, 10:58 AMCaptainBlackQuote:

Originally Posted by**ThePerfectHacker**

so in general not a subgroup.

You might want to consider the group with the following group multiplication

table (which if I have done this right makes a group):

Then: , , , are all subgroups of ,

but is not a group because

is not closed under

RonL - January 22nd 2006, 01:12 PMThePerfectHacker
Thank you CaptainBlack, my only mistake was that I assumed that addition was closed.

Okay now, change the problem, which type of subgroups will form a subgroup?

By the way, I like your group multiplication box, that probably took a long time in Latex code. - January 22nd 2006, 01:28 PMCaptainBlackQuote:

Originally Posted by**ThePerfectHacker**

easy to follow, which is what I did once I had worked out the group table

that I wanted.

RonL - January 22nd 2006, 02:17 PMThePerfectHacker
But it is true that the intersection of subgroups is a subgroup.

- January 24th 2006, 07:50 AMCaptainBlackQuote:

Originally Posted by**ThePerfectHacker**

RonL