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Unusual number : ウィキペディア英語版
Unusual number

In number theory, an unusual number is a natural number ''n'' whose largest prime factor is strictly greater than \sqrt . 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-\sqrt-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....
For any prime p, its multiples less than p² are unusual, that is p, ... (p-1)p, which have a density 1/p in the interval (p,p²).
If we denote the number of unusual numbers less than or equal to ''n'' by ''u''(''n'') then ''u''(''n'') behaves as follows:
Richard Schroeppel stated in 1972 that the asymptotic probability that a randomly chosen number is unusual is ln(2). In other words:
:\lim_ \frac = \ln(2) = 0.693147 \dots\, .
== External links ==

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