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Dirichlet density : ウィキペディア英語版
Dirichlet density

In mathematics, the Dirichlet density (or analytic density) of a set of primes, named after Peter Gustav Lejeune Dirichlet, is a measure of the size of the set that is easier to use than the natural density.
==Definition==

If ''A'' is a subset of the prime numbers, the Dirichlet density of ''A''
is the limit
: \lim_ \frac}}
if it exists. Note that since \sim \log(\frac)} as s\rightarrow 1^+ (see Prime zeta function), this is also equal to
:\lim_\over \log(\frac)}.
This expression is usually the order of the "pole" of
:\prod_{1\over 1-p^{-s}}
at ''s'' = 1, (though in general it is not really a pole as it has non-integral order), at least if the function on the right is a holomorphic function times a (real) power of ''s''−1 near ''s'' = 1. For example, if ''A'' is the set of all primes, the function on the right is the Riemann zeta function which has a pole of order 1 at ''s'' = 1, so the set of all primes has Dirichlet density 1.
More generally, one can define the Dirichlet density of a sequence of primes (or prime powers), possibly with repetitions, in the same way.

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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