Probability mass function
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In probability theory, a probability mass function say p is a real valued function of a discrete variable say x, such that the value p(xk) is the probability of the variable x having the value xk.
In order to qualify, such a function must meet the following criteria:
<math>p(x_{k}) \geq 0 </math> <math> \forall x </math>
<math>\sum_{k} p(x_{k}) = 1 </math>
The collection of pairs ( xk , p(xk) ) is the probability distribution of x.