# Subgroup need not have a left transversal that is also a right transversal

## Statement

It is possible to have a group and a subgroup of such that is *not* a subgroup having a left transversal that is also a right transversal. In other words, there is no subset of that is both a left transversal and a right transversal for in .

## Related facts

### Opposite facts

- Subgroup of finite group has a left transversal that is also a right transversal
- Subgroup of finite index has a left transversal that is also a right transversal

## Proof

### Proof idea

Due to the fact that every group is naturally isomorphic to its opposite group via the inverse map and the resultant fact that left and right coset spaces are naturally isomorphic, there is a global symmetry between the left and right coset space behavior, achieved via the inverse map. This means that if the left looks "bigger" than the right at some place, the right must look correspondingly bigger than the left at other places (specifically, using the inverse map). The hope is to create an example that demonstrates these local disparities between left and right, even though globally, it all balances out.

The goal is to find a right coset that is "big" in the sense of containing more than one left coset. If we can do so, then we will have shown that a left transversal must necessarily pick multiple elements of this right coset, hence cannot be a right transversal.

To construct this kind of subgroup, we need to make it so that left multiplication by an element can be converted into right multiplication by that element, but not the other way around in general. The key idea here is to take a subgroup generated by conjugates by *positive* powers of a certain element on another. This generates the necessary local left-right asymmetry to do our work.

### Proof example

**Construction**:

- Let be a free group on two letters and , so is isomorphic to free group:F2.
- Let be the subgroup of given by . In other words, is generated by and the conjugates of by
*positive*powers of . Note that the isomorphism type of is free group of countable rank.

**Proof that it works**: Explanations of steps to be filled in later.

Step no. | Assertion/construction | Previous steps used | Explanation |
---|---|---|---|

1 | The left coset is contained in the right coset . | ||

2 | The right coset contains the element , which is not an element of . |
||

3 | The right coset equals the double coset , and it is a union of more than one left coset. | ||

4 | Any left transversal of in must necessarily pick more than one element of , hence cannot be a right transversal. |