# Is $(x,y)\rightarrow (-x,-y)$ an inversion transformation?

Does anyone know whether $$(x,y)\rightarrow (-x,-y)$$ is an inversion transformation or not?

I know that the standard inversion (parity) transformation in two dimensions should be something like $$(x,y)\rightarrow (x,-y)$$ which only flips an odd number of spatial coordinates. However, if we have a plane embedded in three-dimensional space and it is located at $$z=0$$, we may have such an inversion transformation for it: $$(x,y,0)\rightarrow (-x,-y,0)$$. Does it imply $$(x,y)\rightarrow (-x,-y)$$ is also a kind of inversion symmetry in two dimensions?

In fact, I found this question when I study the Bernevig-Hughes-Zhang model in physics. Anyone who is familiar with this model may have a better understanding of my question. The Bernevig-Hughes-Zhang model is a famous two-dimensional model for topological insulators with inversion symmetry. It's Bloch Hamiltonian is $$H(\mathbf{k})=\mathbf{\Gamma}\cdot \mathbf{d}$$ where the components of $$\mathbf{\Gamma}$$ are just some Dirac matrices: $$\begin{array}{l} \Gamma^{0}=\mathbb{1} \otimes \mathbb{1} \\ \Gamma^{1}=\tau_{z} \otimes \mathbb{1} \\ \Gamma^{2}=\tau_{y} \otimes \mathbb{1} \\ \Gamma^{3}=\tau_{x} \otimes s_{x} \\ \Gamma^{4}=\tau_{x} \otimes s_{y} \\ \Gamma^{5}=\tau_{x} \otimes s_{z} \end{array}$$ and $$\mathbf{d}$$ reads In this system, the inversion operator is $$P=\tau_z\otimes \mathbb{1}$$. We can easily verify $$PH(k_x,k_y)P^{-1}=H(-k_x,-k_y)$$. However, if we assume the inversion transformation in two dimensions only flips one spatial coordinate (we take $$(x,y)\rightarrow (x,-y)$$ as the example), then the real inversion symmetry restriction for the Bloch Hamiltonian should be $$PH(k_x,k_y)P^{-1}=H(k_x,-k_y)$$, which obviously contradicts what we have gotten. Only if $$(x,y)\rightarrow (-x,-y)$$ is viewed as an inversion transformation, then inversion symmetry restriction becomes $$PH(k_x,k_y)P^{-1}=H(-k_x,-k_y)$$ and everything meets. However, I doubt about this point because as far as I know, a matrix representation of $$P$$ (in any number of dimensions) should have determinant equal to $$−1$$.

You can find this model in Fu and Kane's paper Topological insulators with inversion symmetry.