In Judson Abstract Algebra: Theory and Applications, p. 176, one can read:
Build the permutation representation of $Z_2 \times Z_4$ described in Cayley’s Theorem [...]
I got to this point with Sage (after some pen and paper work):
G = PermutationGroup([
'()',
'(1,2,3,4)(5,6,7,8)',
'(1,3)(2,4)(5,7)(6,8)',
'(1,4,3,2)(5,8,7,6)',
'(1,5)(2,6)(3,7)(4,8)',
'(1,6,3,8)(2,7,4,5)',
'(1,7)(2,8)(3,5)(4,6)',
'(1,8,3,6)(2,5,4,7)'
])
I think this is fine. But, it then asks:
[...] construct the permutation group as a subgroup of a full symmetric group that is generated by exactly two of the eight elements you have already constructed. Hint: which two elements of $Z_2 \times Z_4$ might you use to generate all of $Z_2 \times Z_4$?
What I expect is to find two elements of $Z_2 \times Z_4$ which generate $Z_2 \times Z_4$. But, I have been scratching my head since I simply can't see which elements do this.
I know that elements with the highest order possible in $Z_2 \times Z_4$ will have order $lcm(2, 4) = 4$. So I think I have to choose two such elements. But without even considering the permutation group I built, I cannot get those two elements from pen and paper. To convince myself that something is off, I wrote a sage script:
generated = set()
for element in G:
order = element.order()
assert order in set([1, 2, 4])
if order == 4:
generates = [element ** i for i in range(0, 4)]
generated.update(generates)
It outputs:
{(),
(1,2,3,4)(5,6,7,8),
(1,3)(2,4)(5,7)(6,8),
(1,4,3,2)(5,8,7,6),
(1,6,3,8)(2,7,4,5),
(1,8,3,6)(2,5,4,7)}
which clearly isn't my initial group. What am I doing/understanding wrong here?