Tur\'an inequalities for the plane partition function

Heim, Neuhauser, and Tr\"oger recently established some inequalities for MacMahon's plane partition function $\mathrm{PL}(n)$ that generalize known results for Euler's partition function $p(n)$. They also conjectured that $\mathrm{PL}(n)$ is log-concave for all $n\geq 12.$ We prove this conjecture. Moreover, for every $d\geq 1$, we prove their speculation that $\mathrm{PL}(n)$ satisfies the degree $d$ Tur\'an inequality for sufficiently large $n$. The case where $d=2$ is the case of log-concavity.Comment: 24 pages; Minor revisions based on comments of the referees, to appear in Advances in Mathematic