probability generating function
The mean and variance of X can also be computed using the pgf from
The pgf of the sum X + Y of two independent random variables is given by
and for random sampling, where Z = X1 + … + XN, with N itself a random variable and the Xi are iid, then 
- unit
- unitary matrix
- unitary ratio
- unit circle
- unit cube
- unit matrix
- unit square
- unit vector
- unity
- univalent
- universal covering space
- universal gravitational constant
- universal machine
- universal quantifier
- universal set
- unknot
- unknown
- unstable equilibrium
- upper bound
- upper limit
- upper triangular matrix
- Urysohn's lemma
- utility
- utility function
- V