Recht-Ré conjecture with norms inside the sums

About 6 years old · traced to

Let A1,…,AnA_1,\dots,A_n be positive semidefinite matrices, and let ∥ ⋅ ∥\lVert\,\cdot\,\rVert denote the matrix norm used in the conjecture. For ordered products of length mm, distinguish sums over all index tuples from sums over tuples with distinct indices. Norm-sum Recht-Ré conjecture.

1nm∑1≤j1,…,jm≤n∥Aj1⋯Ajm∥≥(n−m)!n!∑1≤j1,…,jm≤n,\j1,…,jm distinct∥Aj1⋯Ajm∥.\frac{1}{n^m}\sum_{1\leq j_1,\dots,j_m\leq n}\lVert A_{j_1}\cdots A_{j_m}\rVert\geq\frac{(n-m)!}{n!}\sum_{\substack{1\leq j_1,\dots,j_m\leq n,\j_1,\dots,j_m\ \text{distinct}}}\lVert A_{j_1}\cdots A_{j_m}\rVert.

This is a related conjecture attributed to Recht and Ré, with the norms taken termwise rather than after summation. The source does not state whether it has been resolved.

References

Primary source

Zehua Lai and Lek-Heng Lim, “Recht-Ré Noncommutative Arithmetic-Geometric Mean Conjecture is False”, arXiv:2006.01510 (2020).

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