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9 votes

Bounding the trace of a matrix product by the operator norms; generalized Hölder inequality?

I include some information about Hölder's inequality just for completion of details for Mikael's nice answer. The Schatten-$p$ norm of a matrix $X$ is defined as $$\lVert X\rVert_p := \Bigl(\sum\nolim …
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10 votes
Accepted

Norm estimation of identity plus two non-commuting self-adjoint operators

The claim is false. Consider the following matrix argument. \begin{eqnarray*} \|(I+A+B)^{-1}A\| \le 1\quad\Leftrightarrow\quad \begin{bmatrix}I & (I+A+B)^{-1}A \\ A(I+A+B)^{-1} & I \end{bmatrix} \ge …
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4 votes

$\|T\|_2 \le \sqrt{\|T\|_1\|T\|_\infty}$

Sorry, my answer below is only partial, but I thought that it may still be somewhat interesting. As far as I know, this inequality does not have a distinguished name. It is ultimately a consequence …
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17 votes

Matrix trace & norm

Expanding my comment into an answer, which offers a more general result. Theorem (von Neumann). Let $A$ and $B$ be arbitrary $n\times n$ complex matrices. Then, $$|\text{trace}(AB)| \le \sum_{i=1} …
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10 votes
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Bounding the matrix norm of a commutator $[A,B]$ in terms of the norms of $A$ and $B$

A somewhat more general setting, namely, finding the best constant $C_{p,q,r}$ in \begin{equation*} \|AB-BA\|_p \le C_{p,q,r}\|A\|_q\|B\|_r, \end{equation*} for Schatten $p$,$q$,$r$-norms, is studied …
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