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The best possibility of the grand Furuta inequality
Authors:  tarô   Tanahashi
Affiliation:Department of Mathematics, Tohoku College of Pharmacy, Komatsushima, Aoba-ku, Sendai 981-8558, Japan
Abstract:
Let $ A, B in B(H)$ be invertible bounded linear operators on a Hilbert space $H$ satisfying $Oleq B leq A$ , and let $ p, r, s , t $ be real numbers satisfying $ 1 < s, 0 < t < 1 , t leq r , 1 leq p . $ Furuta showed that if $ 0 < alpha leq dfrac{ 1-t+r}{ (p-t)s + r} $, then $ left{ A^{frac{r}{2}} left( A^{ -frac{t}{2}} B^{p} A^{ -frac{t}{2}} right)^{s} A^{frac{r}{2}} right}^{alpha } leq A^{ left{ (p-t)s + r right} alpha } $. This inequality is called the grand Furuta inequality, which interpolates the Furuta inequality $(t=0)$
and the Ando-Hiai inequality ( $ t=1, r = s $ ).

In this paper, we show the grand Furuta inequality is best possible in the following sense: that is, if $ dfrac{ 1-t+r}{ (p-t)s + r} < alpha $, then there exist invertible matrices $A,B$ with $Oleq B leq A$ which do not satisfy $ left{ A^{frac{r}{2}} left( A^{ -frac{t}{2}} B^{p} A^{ -frac{t}{2}} right)^{s} A^{frac{r}{2}} right}^{alpha } leq A^{ left{ (p-t)s + r right} alpha } $.

Keywords:The L"  owner-Heinz inequality, the Furuta inequality, the grand Furuta inequality
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