MathOverflow Asked by Denis Serre on December 3, 2021
Given two normal matrices $A,Bin M_n({mathbb C})$
whose respective spectra are $(alpha_{1},ldots,alpha_{n})$ and
$(beta_{1},ldots,beta_{n})$, is it true that $det(A+B)$ belongs to
the convex hull of the set of numbers
$$prod_{i=1}^n(alpha_i+beta_{sigma(i)}),$$
as $sigma$ runs over the set ${mathfrak S}_n$ of permutations of ${1,ldots,n}$ ?
Nota. It is known (see Exercise 101) that the trace of $AB$ belongs to the convex hull of the
set of numbers
$$sum_{j=1}^nalpha_{j}beta_{sigma(j)},qquadsigmain {mathfrak S}_n.$$
This claim is nothing but the well-known Marcus and de Oliveira conjecture, which has been open since 1973 or earlier.
Reference: Open Problems in Matrix Theory, X. Zhan.
For the simpler case of Hermitian matrices, the claim holds; a slightly more general case seems to be the paper "The validity of the Marcus-de Oliveira conjecture for essentially Hermitian matrices."
PS: you might want to add the "open problem" tag to your question.
Answered by Suvrit on December 3, 2021
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