Named after Boltzmann's Η-theorem, Shannon defined the entropy Η (Greek capital letter eta) of a discrete random variable X {\textstyle X} with possible values { x 1 , … , x n } {\textstyle \left\{x_{1},\ldots ,x_{n}\right\}} and probability mass function P ( X ) {\textstyle \mathrm {P} (X)} as: