Ursell number

The Ursell number is a dimensionless quantity used in fluid mechanics and statistical mechanics, particularly in the context of many-particle systems. It arises in the study of phase transitions and the statistical properties of fluids. Mathematically, the Ursell number \( U \) for a system is defined using the correlation functions of the particles in the system. It is often used to quantify the degree of correlations in a system of interacting particles.