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Golomb–Dickman constant : ウィキペディア英語版
Golomb–Dickman constant
In mathematics, the Golomb–Dickman constant arises in the theory of random permutations and in number theory. Its value is
:\lambda = 0.62432 99885 43550 87099 29363 83100 83724\dots.
Let ''a''''n'' be the average — taken over all permutations of a set of size ''n'' — of the length of the longest cycle in each permutation. Then the Golomb–Dickman constant is
: \lambda = \lim_ \frac.
In the language of probability theory, \lambda n is asymptotically the expected length of the longest cycle in a uniformly distributed random permutation of a set of size ''n''.
In number theory, the Golomb–Dickman constant appears in connection with the average size of the largest prime factor of an integer. More precisely,
:\lambda = \lim_ \frac1n \sum_^n \frac,
where P_1(k) is the largest prime factor of ''k''. So if ''k'' is a ''d'' digit integer, then \lambda d is the asymptotic average number of digits of the largest prime factor of ''k''.
The Golomb–Dickman constant appears in number theory in a different way. What is the
probability that second largest prime factor of ''n'' is smaller than the square root of the largest prime factor of ''n''? Asymptotically, this probability is \lambda.
More precisely,
:\lambda = \lim_ \text\left\
where P_2(n) is the second largest prime factor ''n''.
There are several expressions for \lambda. Namely,
:\lambda = \int_0^\infty e^ dt
where E_1(t) is the exponential integral,
:\lambda = \int_0^\infty \frac dt
and
:\lambda = \int_0^\infty \frac dt
where \rho(t) is the Dickman function.
== See also ==

* Random permutation
* Random permutation statistics

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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