From Wikipedia, the free encyclopedia
- Every odd positive integer (except 1) is 3-rough.
- Every positive integer (except 1) that is congruent to 1 or 5 mod 6 is 5-rough.
The number 1 gives rise to the following "trivial" exceptions:
- The number 1 is not k-rough, for any positive integer k, since it has no prime factors.
- Every positive integer (except 1) is 2-rough, since all its prime factors, being prime numbers, exceed 1.