Let the pair of operators, (H, T), satisfy the weak Weyl relation:
$$T{\rm e}^{-itH}={\rm e}^{-itH}(T+t),$$
where H is self-adjoint and T is closed symmetric. Suppose that g is a real-valued Lebesgue measurable function on \({\mathbb {R}}\) such that \({g\in C^2(\mathbb {R}\backslash K)}\) for some closed subset \({K\subset\mathbb {R}}\) with Lebesgue measure zero. Then we can construct a closed symmetric operator D such that (g(H), D) also obeys the weak Weyl relation.