A matrix of the form $H=\left(\begin{array}{ll}\cosh \alpha & \sinh \alpha \\ \sinh \alpha & \cosh \alpha\end{array}\right)$ for $\alpha \in \mathbb{R}$ defines a hyperbolic rotation of $\mathbb{R}^2$. (a) Prove that all hyperbolic rotations preserve the indefinite quadratic form $q(\mathbf{x})=x^2-y^2$ in the sense that $q(H \mathbf{x})=q(\mathbf{x})$ for all $\mathbf{x}=(x, y)^T \in \mathbb{R}^2$. Observe that ordinary rotations preserve circles $x^2+y^2=a$, while hyperbolic rotations preserve: hyperbolas $x^2-y^2=a$. (b) Are there any other affine transformations of $\mathbb{R}^2$ that preserve the quadratic form $q(\mathbf{x})$ ? Remark. The four-dimensional version of this construction, i.e., affine maps preserving the indefinite Minkowski form $t^2-x^2-y^2-z^2$, forms the geometrical foundation for Einstein's theory of special relativity, [55].