On Differential-Algebraic Equations with Bounded Spectrum in Banach Spaces

The Weierstra{\ss} form for regular DAEs in finite dimensions decouples a linear DAE into an ODE and the nilpotent part of the underlying pencil. Here, we provide necessary and sufficient conditions for the possibility of such a decomposition in the case of DAEs in Banach spaces. Moreover, we consider the larger class of linear operator pencils with bounded spectra and show that the associated homogeneous DAE can be reduced to an ODE and a seemingly simple DAE of the form $\frac d{dt}Tx = x$ with a quasi-nilpotent operator $T$. As examples show, there are cases with only the trivial solution and others with non-trivial solutions. We characterize the existence of $L^\infty$-solutions on the half-axis, $L^2$-solutions on compact time intervals, and analytic solutions.