Sectorial Finitely Generated Dynamical Frames

Abstract

We study frames of unilateral iterations generated by finitely many vectors under an invertible bounded operator $T$. Suppose that the spectrum of $T$ lies in $\{ρe^{it}:0<ρ\le1,\ |t|\le c\}$, where $c<π$, and that its essential spectrum lies in $\{e^{it}:|t|\le c_{\mathrm e}\}$, where $0\le c_{\mathrm e}\le c$. Real powers are defined through the principal logarithm. Our structural result identifies fractional-orbit synthesis, up to an invertible map and a compact perturbation, with synthesis of projected vector-valued exponentials on an arc containing the essential spectrum. Semi-Fredholm properties and Fredholm index therefore transfer. For a uniformly separated temporal set $Λ$, this reduction gives a frame when its one-sided lower uniform density satisfies $D_+^-(Λ)>c_{\mathrm e}/π$ and its logarithmic block density satisfies $L(Λ)>c/π$. Completeness at the latter threshold requires only an invertible operator with spectrum in the corresponding angular sector and a complete integer multiorbit. At $L(Λ)>2c/π$, the complement property is equivalent to a finite condition in channel-cyclic subspaces, with real phase-retrieval consequences. Generalized divided differences admit the same compact reduction and the analogous frame theorem with multiplicities counted. For arbitrary one-point-per-cell sampling with cell length $r$, the sufficient raw-sampling conditions are $rc<π$ and $2rc_{\mathrm e}<π$. Thus, the full range $rc<π$ holds for raw samples when the essential spectrum is contained in $\{1\}$; over the unrestricted class, the sharp universal range is $rc<π/2$. Divided-difference preconditioning restores $rc<π$ for all operators considered and converts exact collisions into logarithmic-derivative data.