Fermionic time-reversal-invariant insulators in two dimension – class AII in the Kitaev table – come in two different topological phases. These are characterized by a \(\mathbb {Z}_2\) -invariant: the Fu–Kane–Mele index. We prove that if two such insulators with different indices occupy regions containing arbitrarily large balls, then the spectrum of the resulting operator fills the bulk spectral gap. Our argument follows a proof by contradiction developed in [16] for quantum Hall systems. It boils down to showing that the \(\mathbb {Z}_2\) -index can be computed only from bulk information in sufficiently large balls. This is achieved via a result of independent interest: a local trace formula for the \(\mathbb {Z}_2\) -index.
