__U__

**Union
**(of sets) If P *and Q *are sets the
union of P *and *Q (written P *U Q) *is the set of all elements that belong to

*P
*or belong to *Q *or belong to both P and *Q.
"P U Q" *is read "P union Q "

**Unique
factorization property **Every composite number can be expressed as the
indicated product of prime

factors
in one and only one way, except for the order of the factors