# Definition A weak topological insulator is a 3D [[Time-Reversal Symmetric Topological Insulator|time-reversal symmetric topological insulator]] characterized by a $Z_2$ [[Z2 Topological Invariant|topological invariant]] of $Z_2 = (0; \nu_1 \nu_2 \nu_3)$ where at least one $\nu_i$ is $1$. # Examples Examples include the $\beta$ phase of Bi$_4$I$_4$. They are poorly studied experimentally since their surface states are not easy to access.