Shannon's equation

Shannon's equation gives a quantitative specification of an information stream produced by an ergodic source:

 H = - K Sigma i = 1 n (pi ld pi) ,

where p denotes the probability of the associated event, H is referred to as the entropy of the information source.

Reference

C. Shannon, W. Weaver, The Mathematical Theory of Communication, University of Illinois Press, 1963, ISBN 0252725484