Discrete Quaternionic (Multi-window) Gabor Systems

The aim of this work is to study (Multi-window) Gabor systems in the space $\ell^2(\mathbb{Z} \times \mathbb{Z}, \mathbb{H})$, denoted by $\mathcal{G}(g,L,M,N)$, and defined by: $$\left\{ (k_1,k_2)\in \mathbb{Z}^2\mapsto e^{2\pi i \frac{m_1}{M}k_1} g_l(k - nN) e^{2\pi j \frac{m_2}{M}k_2} \right\}_{l \in \mathbb{N}_L, (m_1, m_2) \in \mathbb{N}_M^2, n \in \mathbb{Z}^2},$$ where, $L,M,N$ are positive integers, $i,j$ are the imaginary units in the quaternion algebra, and $\{g_l\}_{l \in \mathbb{N}_L} \subset \ell^2(\mathbb{Z} \times \mathbb{Z}, \mathbb{H})$. Special emphasis is placed on the case where the sequences $g_l$ are real-valued. The questions addressed in this work include the characterization of quaternionic Gabor systems that form frames, the characterization of those that are orthonormal bases, and the admissibility of such systems. We also explore necessary and/or sufficient conditions for Gabor frames. The issue of duality is also discussed. Furthermore, we study the stability of these systems.