Abstract
This paper concerns the facial geometry of the set of \(n \times n\) correlation matrices. The main result states that almost every set of r vertices generates a simplicial face, provided that \(r \le \sqrt{\mathrm {c} n}\), where \(\mathrm {c}\) is an absolute constant. This bound is qualitatively sharp because the set of correlation matrices has no simplicial face generated by more than \(\sqrt{2n}\) vertices.
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Notes
In our parameter regime, it is unlikely that any vertex of \(\mathscr {E}_n\) is chosen more than once, so this model is not substantially different from drawing vertices without replacement.
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Acknowledgements
The author thanks Richard Küng and Benjamin Recht for helpful conversations related to this work. This research was partially supported by ONR award N00014-11-1002 and the Gordon & Betty Moore Foundation.
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Tropp, J.A. Simplicial Faces of the Set of Correlation Matrices. Discrete Comput Geom 60, 512–529 (2018). https://doi.org/10.1007/s00454-017-9961-0
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DOI: https://doi.org/10.1007/s00454-017-9961-0