 Zwanzig projection operator

The Zwanzig projection operator is a mathematical tool used in the field of statistical mechanics and nonequilibrium thermodynamics to derive reduced descriptions of many-body systems. Named after Robert Zwanzig, it is particularly useful for studying systems with a large number of degrees of freedom, allowing one to focus on the relevant variables while ignoring others. The basic idea behind the Zwanzig projection operator is to split the total phase space of a system into "relevant" and "irrelevant" parts.