In number theory, an unusual number is a natural number n whose largest prime factor is strictly greater than (sequence A064052 in OEIS). All prime numbers are unusual.
A k-smooth number has all its prime factors less than or equal to k, therefore, an unusual number is non--smooth.
The first few unusual numbers are 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17, 19, 20, 21, 22, 23, 26, 28, 29, 31, 33, 34, 35, 37, 38, 39, 41, 42, 43, 44, 46, 47, 51, 52, 53, 55, 57, 58, 59, 61, 62, 65, 66, 67....
The first few non-prime unusual numbers are 6, 10, 14, 15, 20, 21, 22, 26, 28, 33, 34, 35, 38, 39, 42, 44, 46, 51, 52, 55, 57, 58, 62, 65, 66, 68, 69, 74, 76, 77, 78, 82, 85, 86, 87, 88, 91, 92, 93, 94, 95, 99, 102....
If we denote the number of unusual numbers less than or equal to n by u(n) then u(n) behaves as follows:
| n | u(n) | u(n) / n |
| 10 | 6 | 0.6 |
| 100 | 67 | 0.67 |
| 1000 | 715 | 0.715 |
| 10000 | 7319 | 0.7319 |
| 100000 | 70128 | 0.70128 |
Richard Schroeppel proved in 1972 that the asymptotic probability that a randomly chosen number is unusual is ln(2). In other words:
Famous quotes containing the words unusual and/or number:
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—Raymond Chandler (1888–1959)
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—Susan Lapinski (20th century)