Generalised Krein-Feller operators and gap diffusions via transformations of measure spaces

We consider the generalised Krein-Feller operator $\Delta_{\nu, \mu}$ with respect to compactly supported Borel probability measures $\mu$ and $\nu$ with the natural restrictions that $\mu$ is atomless, the supp$(\nu)\subseteq$supp$(\mu)$ and the atoms of $\nu$ are embedded in the supp$(\mu)$. We show that the solutions of the eigenvalue problem for $\Delta_{\nu, \mu}$ can be transferred to the corresponding problem for the classical Krein-Feller operator $\Delta_{\nu \circ F_{\mu}^{-1}, \Lambda}$ with respect to the Lebesgue measure $\Lambda$ via an isometric isomorphism determined by the distribution function $F_\mu$ of $\mu$. In this way, we obtain a new characterisation of the upper spectral dimension and consolidate many known results on the spectral asymptotics of Krein-Feller operators. We also recover known properties of and connections to generalised gap diffusions associated to these operators.