Sharp Conditioning for Matrix Recovery by Finite Affine Orbits

Abstract

Let \(q=p^h\) be an odd prime power, and let the affine group \(\mathbb F_q\rtimes\mathbb F_q^\times\) act through its canonical \((q-1)\)-dimensional irreducible representation. Qualitative matrix recovery for these rank-one orbits is known. We determine sharp lower singular-value bounds for explicit real generating windows. First, we compute the exact least singular value for every nonnegative two-level window over \(\mathbb F_{p^h}\) with \(h\geq2\), and identify the unique optimizer when \(q-1\geq10\). The resulting conditioning stays bounded away from zero on every fixed odd-characteristic tower and is within an explicit characteristic-dependent factor of the best possible value over all real windows. For every \(q=3^h\), \(h\geq2\), we construct a real three-level absolute-trace window (constant on the fibers of \(\operatorname{Tr}_{\mathbb F_q/\mathbb F_3}\)) whose least singular value is \[ \frac{q(\sqrt2-1)}{q(2-\sqrt2)-1}>\frac1{\sqrt2}. \] For the full class of real windows constant on the three trace classes, we reduce the least singular value to four scalar expressions and a symmetric \(2\times2\) matrix. This yields the exact global optimum and all equality cases: the displayed trace window is uniquely optimal up to global sign and interchange of the two nonzero trace classes. Thus zero trace mean follows from optimality. The proof combines an explicit sum-of-squares identity, uniform quadratic-form certificates, and a finite-geometric block decomposition of the orbit measurement operator.