Hugo van Dam and the dynamic adjoint function

This paper starts with some subjective reflections on the properties, interpretation and the use of the adjoint equation in neutron transport theory. This is used as a setting to put into perspective the seminal contribution of Hugo van Dam to the development of neutron noise diagnostics by introducing the use of the dynamic adjoint function. The significance of this development, although clear to most researchers in the field, has not been discussed formally and proper credit has not been given to its significance. It is the intention of this paper to make up for this fact. Some personal reminiscences on other areas of Hugoʹs scientific activity are also given at the end.