Advances on the Bessis-Moussa-Villani Trace Conjecture

A long-standing conjecture asserts that the polynomial $$p(t) = \text{Tr}[(A+tB)^m]$$ has nonnegative coefficients whenever $m$ is a positive integer and $A$ and $B$ are any two $n \times n$ positive semidefinite Hermitian matrices. The conjecture arises from a question raised by Bessis, Moussa, and Villani (1975) in connection with a problem in theoretical physics. Their conjecture, as shown recently by Lieb and Seiringer, is equivalent to the trace positivity statement above. In this paper, we derive a fundamental set of equations satisfied by $A$ and $B$ that minimize or maximize a coefficient of $p(t)$. Applied to the Bessis-Moussa-Villani (BMV) conjecture, these equations provide several reductions. In particular, we prove that it is enough to show that (1) it is true for infinitely many $m$, (2) a nonzero (matrix) coefficient of $(A+tB)^m$ always has at least one positive eigenvalue, or (3) the result holds for singular positive semidefinite matrices. Moreover, we prove that if the conjecture is false for some $m$, then it is false for all larger $m$.