The probability mass function of a discrete random variable 
is the function 
. It assigns the probability
of each possible value and satisfies
 |
and
 |
where the sum is over the possible values of 
.
Unlike a probability density function, a probability mass function gives point probabilities
directly, without integration. For several discrete
random variables, the joint probability mass function
is 
(Cover and Thomas 2006, pp. 13 and 16).
