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Isomorphic groups question. Please help!!

Hi guys, I've been set this question and I'm a bit stuck.

Find which pairs of the following groups are isomorphic:
(i) the dihedral group D6 of symmetries of a regular hexagon
(ii) Z6
(iii) Z12
(iv) the group of rotations of a regular hexagon
(v) the group of rotations of a regular tetrahedron.

I've worked out that Z6 and the rotations of a hexagon are isomorphic from comparing the Cayley tables (and they're both cyclic of order 6) but I'm not sure if there's anymore.
I know that the rotations of a tetrahedron has order 12 but I'm not sure if it's cyclic and I wouldn't know where to start with a Cayley table.

Thanks in advance!
Reply 1
Original post by Viggerz
Hi guys, I've been set this question and I'm a bit stuck.

Find which pairs of the following groups are isomorphic:
(i) the dihedral group D6 of symmetries of a regular hexagon
(ii) Z6
(iii) Z12
(iv) the group of rotations of a regular hexagon
(v) the group of rotations of a regular tetrahedron.

I've worked out that Z6 and the rotations of a hexagon are isomorphic from comparing the Cayley tables (and they're both cyclic of order 6) but I'm not sure if there's anymore.
I know that the rotations of a tetrahedron has order 12 but I'm not sure if it's cyclic and I wouldn't know where to start with a Cayley table.

Thanks in advance!


Given your instincts that the others aren't isomorphic, you'd be best looking for algebraic differences between them; two groups won't be isomorphic if;

they're of different orders.
they're not both commutative.
they're not both cyclic.
they have different numbers of elements of (a given) order n.

That should be enough to separate out these groups.
Reply 2
Original post by RichE
Given your instincts that the others aren't isomorphic, you'd be best looking for algebraic differences between them; two groups won't be isomorphic if;

they're of different orders.
they're not both commutative.
they're not both cyclic.
they have different numbers of elements of (a given) order n.

That should be enough to separate out these groups.


D6: order 12, commutative, non cyclic
Z6: order 6, commutative, cyclic
Z12: order 12, commutative, cyclic
rotations of hexagon: order 6, commutative, cyclic
rotations of tetrahedron: order 12, commutative, ?

and this is where I get stuck. Is what I've said correct so far?

Thanks for your help :smile:
Reply 3
Original post by Viggerz
D6: order 12, commutative, non cyclic
Z6: order 6, commutative, cyclic
Z12: order 12, commutative, cyclic
rotations of hexagon: order 6, commutative, cyclic
rotations of tetrahedron: order 12, commutative, ?

and this is where I get stuck. Is what I've said correct so far?

Thanks for your help :smile:


D6 is not commutative, and nor is the rotational group of a tetrahedron.
Reply 4
Original post by RichE
D6 is not commutative, and nor is the rotational group of a tetrahedron.


Ok, thank you.

In that case, I think that the only isomorphic pair is Z6 and the rotations of a regular hexagon.
Reply 5
Original post by Viggerz
Ok, thank you.

In that case, I think that the only isomorphic pair is Z6 and the rotations of a regular hexagon.


That's correct, but you still need some means of distinguishing between the two non-commutative groups of order 12.

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