jagged number | unusual number

Finch (2001, 2003) defines a k-rough (or k-jagged) number to be positive integer all of whose prime factors are greater than or equal to k. Greene and Knuth define "unusual numbers" as numbers n whose greatest prime factor is greater than or equal to sqrt(n), and these number are dubbed "sqrt(n)-rough" or "sqrt(n)-jagged" by Finch (2001, 2003). The first few unusual numbers are 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, ... (OEIS A063538), which turn out to not be so unusual after all (Greene and Knuth 1990, Finch 2001). The first few "usual" numbers are then 8, 12, 16, 18, 24, 27, 30, ... (OEIS A063539).

greatest prime factor | round number | semiprime | smooth number