Named in honor of Gen-Ichiro Hori (b. 1930), professor emeritus at Tokyo University who gave lectures on celestial mechanics at Tokyo University, Yale University and the University of Texas at Austin. He developed the famous perturbation theory based on Lie transformations that has been adopted by many scientists to solve a set of differential equations by perturbation methods. In fact, his publication on this in 1966 is the most frequently cited paper during the 50-year history of the Publication of the Astronomical Society of Japan. Name proposed by the discoverers, following a suggestion by S. Murayama, K. Hurukawa and A. Fujii, and endorsed by Y. Kozai, K. Aksnes and B. G. Marsden.