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Minimal scalings and structural properties of scalable frames. (English) Zbl 1378.42017
Summary: For a unit-norm frame $$F=\{f_i\}^k_{i=1} \mathrm{in} \mathbb{R}^{n}$$, a scaling is a vector $$c= (c(1),\ldots,c(k))\in \mathbb{R}^{k}_{\geqslant 0}$$ such that $$\{\sqrt{c(i)f_i}\}^k_{i=1}$$ is a Parseval frame in $$\mathbb{R}^{n}$$. If such a scaling exists, $$F$$ is said to be scalable. A scaling $$c$$ is a minimal scaling if $$\{f_i: c(i)>0\}$$ has no proper scalable subframe. In this paper, we provide an algorithm to find all possible contact points for the John’s decomposition of the identity by applying the b-rule algorithm to a linear system which is associated with a scalable frame. We also give an estimate of the number of minimal scalings of a scalable frame. We provide a characterization of when minimal scalings are affinely dependent. Using this characterization, we can conclude that all strict scalings $$c= (c(1),\ldots,c(k)) \in \mathbb{R}^{k}_{>0}$$ of $$F$$ have the same structural property. That is, the collections of all tight subframes of strictly scaled frames are the same up to a permutation of the frame elements. We also present the uniqueness of the orthogonal partitioning property of any set of minimal scalings, which provides all possible tight subframes of a given scaled frame.
##### MSC:
 42C15 General harmonic expansions, frames 05B20 Combinatorial aspects of matrices (incidence, Hadamard, etc.) 15A03 Vector spaces, linear dependence, rank, lineability
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