Vishwakarma, PK (2023) Positivity preservers forbidden to operate on diagonal blocks. In: Transactions of the American Mathematical Society, 376 (8). pp. 5261-5279.
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Abstract
The question of which functions acting entrywise preserve positive semidefiniteness has a long history, beginning with the Schur product theorem [Crelle 1911], which implies that absolutely monotonic functions (i.e., power series with nonnegative coefficients) preserve positivity on matrices of all dimensions. A famous result of Schoenberg and of Rudin [Duke Math. J. 1942, 1959] shows the converse: there are no other such functions. Motivated by modern applications, Guillot and Rajaratnam [Trans. Amer. Math. Soc. 2015] classified the entrywise positivity preservers in all dimensions, which act only on the off-diagonal entries. These two results are at “opposite ends”, and in both cases the preservers have to be absolutely monotonic. We complete the classification of positivity preservers that act entrywise except on specified “diagonal/principal blocks”, in every case other than the two above. (In fact we achieve this in a more general framework.) This yields the first examples of dimension-free entrywise positivity preservers - with certain forbidden principal blocks - that are not absolutely monotonic. c 2023 American Mathematical Society.
| Item Type: | Journal Article |
|---|---|
| Publication: | Transactions of the American Mathematical Society |
| Publisher: | American Mathematical Society |
| Additional Information: | The copyright for this article belongs to the Author. |
| Department/Centre: | Division of Physical & Mathematical Sciences > Mathematics |
| Date Deposited: | 24 Nov 2023 05:28 |
| Last Modified: | 24 Nov 2023 05:28 |
| URI: | https://eprints.iisc.ac.in/id/eprint/83244 |
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