Parity Group is Group

Theorem
The parity group is in fact a group.

Proof
We can completely describe the parity group by showing its Cayley table:


 * $\begin{array}{r|rr}

\left({\left\{{1, -1}\right\}, \times}\right) & 1 & -1\\ \hline 1 & 1 & -1 \\ -1 & -1 & 1 \\ \end{array} \qquad \begin{array}{r|rr} \left({\Z_2, +_2}\right) & \left[\!\left[{0}\right]\!\right]_2 & \left[\!\left[{1}\right]\!\right]_2\\ \hline \left[\!\left[{0}\right]\!\right]_2 & \left[\!\left[{0}\right]\!\right]_2 & \left[\!\left[{1}\right]\!\right]_2 \\ \left[\!\left[{1}\right]\!\right]_2 & \left[\!\left[{1}\right]\!\right]_2 & \left[\!\left[{0}\right]\!\right]_2 \\ \end{array}$

It is easily checked that the four group axioms hold for both instances.

As there is only one group of order 2, up to isomorphism, from Prime Group is Cyclic, we see that both examples of the parity group are isomorphic to $C_2$.