Stone formula

For a self-adjoint operator $A$, fix the resolvent convention $R(z)=(A-zI{)}^{-1}$. For real $a<b$, Stone formula states the strong-operator limitEquivalently the integrand is ${\pi }^{-1}\mathrm{Im}R(t+iϵ)$. The endpoint halves are necessary when there are eigenvalues at the endpoints; if neither endpoint has a spectral atom, the limit is simply the interval spectral projection.^[ref]

To see the limit, use the spectral theorem to replace the resolvent difference by the scalar kernelIt lies in $[0,1]$. Its arctangent primitive tends to one inside $(a,b)$, zero outside $[a,b]$, and one half at each endpoint. For every vector $x$, dominated convergence applied to $\int |F_{ϵ}-F_0{|}^{2}d{\mu }_x$ proves convergence in norm after applying the operators to $x$. This explains both the strong rather than necessarily operator-norm limit and the half weights.