redundancy of two-sidedness in definition of group
In the definition of group, one usually supposes that there is a two-sided identity element and thatany element has a two-sided inverse
(cf. group (http://planetmath.org/Group)).
The group may also be defined without the two-sidednesses:
A group is a pair of a non-empty set and its associative binary operation such that
1) the operation has a right identity element ;
2) any element of has a right inverse .
We have to show that the right identity is also a left identityand that any right inverse is also a left inverse.
Let the above assumptions on be true. If is the rightinverse of an arbitrary element of , the calculation
shows that it is also the left inverse of . Using this result,we then can write
whence is a left identity element, too.