Coarse comparison of Bessel families

Abstract

We classify Bessel families in separable Hilbert spaces under coarse equivalence relations: $f$ and $\tilde f$ are equivalent if the seminorms $\|T_f\,\cdot\|$ and $\|T_{\tilde f}\,\cdot\|$ of their analysis operators are mutually dominated, up to constants, after pullback along operators $w,\tilde w$ of a prescribed class $\mathcal O$. In contrast with orbit relations such as unitary or similarity equivalence, these relations ignore redundancy and the geometry of the index set, and compare frames, nonframes and compact families on the same footing. They are level $-1$ of the comparison categories of the companion article, and we compute the levels $0$ and $k\ge1$: they are trivial for bounded, injective, bounded below, isometric and upper semi-Fredholm comparison operators, and for invertible or unitary ones level $1$ is a stabilizer-type group of the family. For bounded $\mathcal O$ the classes are given by the singular numbers up to constants; injective operators add the kernel dimension, bounded below operators or isometries add upper semi-Fredholmness, and invertible, unitary or surjective operators give the kernel dimension and the dyadic multiplicity sequence of the spectrum up to bounded displacement on the logarithmic scale, which splits each remaining class into continuum many. Upper semi-Fredholm operators give only three noncompact classes, whereas Fredholm operators give continuum many, so that the Fredholm relation is strictly finer. For each relation we determine exactly which families are equivalent to an orthonormal basis, and we compute an operator Banach--Mazur distance, $\log(\sup_ns_n/\tilde s_n\cdot\sup_n\tilde s_n/s_n)$. As applications, we obtain a relative Paley--Wiener stability theorem for Bessel families and the invariance of Schatten membership and of $\int\|f\|^2$ under coarse equivalence.