In thermodynamics, the H-theorem, introduced by Ludwig Boltzmann in 1872, describes the increase in the entropy of an ideal gas in an irreversible process, by considering the Boltzmann equation. It appears to predict an irreversible increase in entropy, despite microscopically reversible dynamics. This has led to much discussion. Quantum mechanical H-theorem The H-function's the function: » H= sum_i p_i ln p_i, , where the summation runs over all the possible distinct states of the system, and… (More on H-theorem)