Character annihilator ($H^{\perp }$)

For a nontrivial additive character $\psi$ on a local field, the character annihilator of an additive subgroup $H$ is $H^{\perp }=\{y:\psi (xy)=1\text{ for all }x\in H\}$. If ${\mathcal{O}}_F^{\perp }={\pi }^c{\mathcal{O}}_F$, then $({\pi }^n{\mathcal{O}}_F{)}^{\perp }={\pi }^{c-n}{\mathcal{O}}_F$. This is a character pairing construction, distinct from the annihilator of a module.