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Sharp nonzero lower bounds for the Schur product theorem.
- Source :
- Proceedings of the American Mathematical Society; Dec2021, Vol. 149 Issue 12, p5049-5063, 15p
- Publication Year :
- 2021
-
Abstract
- By a result of Schur [J. Reine Angew. Math. 140 (1911), pp. 1–28], the entrywise product M ∘ N of two positive semidefinite matrices M,N is again positive. Vybíral [Adv. Math. 368 (2020), p. 9] improved on this by showing the uniform lower bound M ∘ M ≥ E<subscript>n</subscript>/n for all n × n real or complex correlation matrices M, where E<subscript>n</subscript> is the all-ones matrix. This was applied to settle a conjecture of Novak [J. Complexity 15 (1999), pp. 299–316] and to positive definite functions on groups. Vybíral (in his original preprint) asked if one can obtain similar uniform lower bounds for higher entrywise powers of M, or for M ∘ N when N ≠ M, M. A natural third question is to ask for a tighter lower bound that does not vanish as n → ∞, i.e., over infinite-dimensional Hilbert spaces. In this note, we affirmatively answer all three questions by extending and refining Vybíral's result to lower-bound M ∘ N, for arbitrary complex positive semidefinite matrices M,N. Specifically: we provide tight lower bounds, improving on Vybíral's bounds. Second, our proof is 'conceptual' (and self-contained), providing a natural interpretation of these improved bounds via tracial Cauchy–Schwarz inequalities. Third, we extend our tight lower bounds to Hilbert–Schmidt operators. As an application, we settle Open Problem 1 of Hinrichs–Krieg–Novak–Vybíral [J. Complexity 65 (2021), Paper No. 101544, 20 pp.], which yields improvements in the error bounds in certain tensor product (integration) problems. [ABSTRACT FROM AUTHOR]
- Subjects :
- SCHWARZ inequality
COMPLEX matrices
TENSOR products
HILBERT space
MATHEMATICS
Subjects
Details
- Language :
- English
- ISSN :
- 00029939
- Volume :
- 149
- Issue :
- 12
- Database :
- Complementary Index
- Journal :
- Proceedings of the American Mathematical Society
- Publication Type :
- Academic Journal
- Accession number :
- 153034039
- Full Text :
- https://doi.org/10.1090/proc/15555