Solution
= Solution
Invariance for every $x$ requires
$$
\boxed{\Lambda^T\eta\Lambda=\eta},
\qquad
\eta_{\rho\sigma}\Lambda^\rho{}_\mu\Lambda^\sigma{}_\nu
=\eta_{\mu\nu}.
$$
Thus $G=O(1,3)$, the <Lorentz group>; its identity component is $SO^+(1,3)$.