We want to select from a set of hash functions H with reasonable certainty that
the above property is true.
Thus, the number of functions f in H such that h(x) = h(y) for
x,y U must satisfy
Definition: A universal collection of hash functions H
contains exactly hash functions such that h(x) = h(y)
for x,y U.
is a universal collection of hash functions.
Thus, we want to randomly select ``a'' each time.