Preface to the special issue dedicated to James Montaldi

In August 2018 we held a conference in Guanajuato, Mexico, where several collaborators and students of James Montaldi had the privilege to homage his scientific contributions. James is best known as an expert in singularity and bifurcation theory, who is a world reference in equivariant Hamiltonian systems, and has an exceptional talent to turn symmetries into insightful theorems about the dynamics of mechanical systems. His unusual combination of mathematical depth, modesty, integrity and friendly personality, make him a greatly esteemed member of the extended geometric mechanics community. The programme of this exciting meeting, which lasted one week, incorporated some of the many areas in which James has worked over the years. During the conference, we heard testimonials of James’ mathematical sharpness, kindness and generosity as a supervisor and collaborator, accompanied with many nostalgic references of a meeting that he organised in Peyresq twenty years ago. This issue of the Journal of Geometric Mechanics is a continuation of our celebration of James’ career and of our appreciation of having him as a teacher, colleague and friend. Out of the many topics of the conference, the following two allow us to better understand his background and expertise, and to describe a noteworthy contribution to symmetric Hamiltonian systems that he made at an early stage of his career: