English

On norm inequalities related to the geometric mean

Functional Analysis 2022-10-26 v1

Abstract

Let AiA_i and BiB_i be positive definite matrices for all i=1,,m.i=1,\cdots,m. It is shown that i=1m(Ai2Bi2)r1((i=1mAi)pr2(i=1mBi)pr(i=1mAi)rp2)1p1,\left|\left|\sum_{i=1}^m(A_i^2\sharp B_i^2)^r\right|\right|_1\leq\left|\left|\left(\left(\sum_{i=1}^mA_i\right)^{\frac{pr}{_2}}\left(\sum_{i=1}^mB_i\right)^{pr}\left(\sum_{i=1}^mA_i\right)^{\frac{rp}{_2}}\right)^{\frac{1}{p}}\right|\right|_1,for all p>0p>0 and for all r1.r\geq1. We conjecture this inequality is also true for all unitarily invariant norms. We give an affirmative answer to the case of m=2,m=2, p1p\geq1, r1r\geq1 and for all unitarily invariant norms. In other words, it is shown that (A2B2)r+(C2D2)r((A+C)rp2(B+D)rp(A+C)rp2)1p,\left|\left|\left|\left(A^{^2}\sharp B^{^2}\right)^{r}+\left(C^{^2}\sharp D^{^2}\right)^{r}\right|\right|\right|\leq \left|\left|\left|\left(\left(A+C\right)^{^\frac{rp}{_2}}\left(B+D\right)^{{rp}}\left(A+C\right)^{^\frac{rp}{_2}}\right)^{\frac{1}{_p}}\right|\right|\right|,for all unitarly invariant norms, for all p1p\geq1 and for all r1r\geq1, where A,B,C,DA,B,C,D are positive definite matrices. This gives an affirmative answer to the conjecture posed by Dinh, Ahsani and Tam in the case of m=2m=2. The preceding inequalities directly lead to a recent result of Audenaert \cite{ANIFP}.

Keywords

Cite

@article{arxiv.2210.14023,
  title  = {On norm inequalities related to the geometric mean},
  author = {Shaima'a Freewan and Mostafa Hayajneh},
  journal= {arXiv preprint arXiv:2210.14023},
  year   = {2022}
}