A number can be shown to be a Riesel number by exhibiting a "covering set": a set of prime numbers that will divide any member of the sequence, so called because it is said to "cover" that sequence.
A number can be shown to be a Riesel number by exhibiting a "covering set": a set of prime numbers. This set of prime numbers will divide any member of the sequence. The sequence is so called. It is said to "cover" that sequence.