On an integral formula for Fredholm determinants related to pairs of spectral projections

We consider Fredholm determinants of the form identity minus product of spectral projections corresponding to isolated parts of the spectrum of a pair of self-adjoint operators. We show an identity relating such determinants to an integral over the spectral shift function in the case of a rank-one perturbation. More precisely, we prove $$-\ln (\det (\mathbf{1}-{\mathbf{1}}_I(A){\mathbf{1}}_{\mathbb{R}\mathrm{\backslash }I}(B){\mathbf{1}}_I(A)\left)\right)={\int }_I\text{d}x{\int }_{\mathbb{R}\mathrm{\backslash }I}\text{d}y\frac{\xi (x)\xi (y)}{(y-x{)}^2},$$ where ${\mathbf{1}}_J(\cdot )$ denotes the spectral projection of a self-adjoint operator on a set $J\in $\text{Borel}$(\mathbb R)$\mathrm{.}Theoperators$A$and$B$ are self-adjoint, bounded from below and differ by a rank-one perturbation and $\xi$ denotes the corresponding spectral shift function. The set $I$ is a union of intervals on the real line such that its boundary lies in the resolvent set of $A$ and $B$ and such that the spectral shift function vanishes there i.e. $I$ contains isolated parts of the spectrum of $A$ and $B$. We apply this formula to the subspace perturbation problem.