The paper “Finite-density lattice QCD and sign problem: current status and open problems“, a 91-page review article by Dr Keitaro Nagata has been published in Progress in Particle and Nuclear Physics (link here). The article was originally written by Dr Nagata in 2019 in the Japanese language and he died before it could be finished. It was translated, finished, and edited by Masanori Hanada and Etsuko Itou (Kyoto). The article is a review of the simulation approaches to quantum chromodynamics at high density environment. High-density QCD has many important applications, such as the physics inside neutron stars. While more experimental or observational tools emerge (such as the gravitational wave from the neutron star-black hole merger), first-principle study from quantum chromodynamics is still very difficult because the sign problem prevents us from applying Monte Carlo methods. There are several interesting proposals in the past two decades, which are discussed in this review. The image below shows the title page of the article as well as photos of Dr Nagata.