Irregular sampling for hyperbolic secant type functions

Abstract

We study Gabor frames in the case when the window function is of hyperbolic secant type, i.e., $g(x) = (e^{ax}+e^{-bx})^{-1}$, ${\rm Re}\,a, {\rm Re}\,b>0$. A criterion for half-irregular sampling is obtained: for a separated $\Lambda\subset\mathbb{R}$ the Gabor system $\mathcal{G}(g, \Lambda \times \alpha\Z)$ is a frame in $L^2(\R)$ if and only if $D^-(\Lambda) >\alpha$ where $D^-(\Lambda)$ is the usual (Beurling) lower density of $\Lambda$. This extends a result by Gr\"ochenig, Romero, and St\"ockler which applies to the case of a standard hyperbolic secant. Also, a full description of complete interpolating sequences for the shift-invariant space generated by $g$ is given.