Bases of complex exponentials with restricted supports

The complex exponentials with integer frequencies form a basis for the space of square integrable functions on the unit interval. We analyze whether the basis property is maintained if the support of the complex exponentials is restricted to possibly overlapping subsets of the unit interval. We show, for example, that if $S_1,\dots ,S_K\subset [0,1]$ are finite unions of intervals with rational endpoints that cover the unit interval, then there exists a partition of $\mathbb{Z}$ into sets ${\mathrm{\Lambda }}_1,\dots ,{\mathrm{\Lambda }}_K$ such that ${\bigcup }_{k=1}^K\{e^{2\pi i\lambda (\cdot )}{\chi }_{S_k}:\lambda \in {\mathrm{\Lambda }}_k\}$ is a Riesz basis for $L^2[0,1]$. Here, ${\chi }_S$ denotes the characteristic function of $S$.