Singular values of $X+iY$ where $X$ and $Y$ are Hermitian Lets have two Hermitian $n\times n$ matrices $X$ and $Y$.
Are there any known properties of the singular values of
$$Z = X + i Y.$$
I am the most interested in bounding from above a few first singular values of $Z$ by the eigenvalues of $X$ and $Y$. And sth that is stronger than:
$$\sum_{i=1}^k \sigma_i^2(Z)\leq \sum_{i=1}^k \left( \lambda_i^2(X)+\lambda_i^2(Y)+\lambda_i(i[X,Y]) \right)$$
for $1\leq k \leq n$ and (singular/eigen)values sorted in the decreasing order.
 A: Some results in this direction that you might find useful are listed below.

Theorem (Bhatia and Kittaneh). Let $X$, $Y$, and $Z$ be as in the question above. Then,
  \begin{equation*}
\| (X^2+Y^2)^{1/2} \|_p \le \|Z\|_p \le 2^{1/2-1/p}\| (X^2+Y^2)^{1/2} \|_p,
\end{equation*}
  where $2 \le p \le \infty$, and $\|\cdot\|_p$ denotes the Schatten-$p$ norm. The inequality above gets reversed for $1\le p \le 2$. Also, these inequalities are sharp.

Even more directly relevant is the following theorem that discusses majorization of singular values of $X+Y$ by those of $Z$.

Theorem (Bhatia and Kittaneh). Let $X$, $Y$, and $Z$ be as in the question. Then
  \begin{equation*}
\sigma(X+Y)\quad \prec_w\quad \sqrt{2}\sigma(Z)
\end{equation*}
  If $X$ is psd, then the above weak majorization can be replaced by weak log-majorization, that is,
  \begin{equation*}
\prod_{j=1}^k \sigma_j(X+Y) \le \prod_{j=1}^k\sqrt{2}\sigma_j(Z).
\end{equation*}
  Finally, if both $X$ and $Y$ are psd, then we have even stronger inequalities:
  \begin{equation*}
  \sigma_j(X+Y) \le \sqrt{2}\sigma_j(Z)\quad 1 \le j \le n.
\end{equation*}

Bhatia and Kittaneh also discuss some applications of the above theorem to commutator inequalities.
References


*

*R. Bhatia and F. Kittaneh. "The singular values of $A+B$ and $A+iB$." Linear Algebra and its Applications, 431(2009), pp. 1502-1508.

*R. Bhatia and F. Kittaneh. "Cartesian decompositions and Schatten norms." Linear Algebra and its Applications, 318(2000), pp. 109--116.
