Possible Duplicate:
Commutator subgroup does not consist only of commutators?
I am looking for an simple example (i.e. for undergraduate students) of group whose commutators $xyx^{1}y^{1}$ do not form a subgroup.
I am looking for an simple example (i.e. for undergraduate students) of group whose commutators $xyx^{1}y^{1}$ do not form a subgroup. 

marked as duplicate by HJRW, Mark Sapir, Chris Godsil, Tony Huynh, Richard Kent Jan 11 '12 at 12:54This question was marked as an exact duplicate of an existing question. 

