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Arithmetic function : ウィキペディア英語版
Arithmetic function
In number theory, an arithmetic, arithmetical, or number-theoretic function is a real or complex valued function ''f''(''n'') defined on the set of natural numbers (i.e. positive integers) that "expresses some arithmetical property of ''n''".〔Hardy & Wright, intro. to Ch. XVI〕
An example of an arithmetic function is the non-principal character (mod 4) defined by
:
\chi(n) =
\left(\frac\right)=
\begin
\;\;\,0 & \text n \text, \\
\;\;\, 1 & \text n \equiv 1 \mod 4, \\
-1 & \text n \equiv 3 \mod 4,
\end
   
where (\tfrac)\ is the Kronecker symbol.
To emphasize that they are being thought of as functions rather than sequences, values of an arithmetic function are usually denoted by ''a''(''n'') rather than ''a''''n''.
There is a larger class of number-theoretic functions that do not fit the above definition, e.g. the prime-counting functions. This article provides links to functions of both classes.
==Notation==
\sum_p f(p)\;   and   \prod_p f(p)\;   mean that the sum or product is over all prime numbers:
:\sum_p f(p) = f(2) + f(3) + f(5) + \cdots     \prod_p f(p)= f(2)f(3)f(5)\cdots.
Similarly,   \sum_ f(p^k)\;   and   \prod_ f(p^k)\;   mean that the sum or product is over all prime powers with strictly positive exponent (so 1 is not included):
:\sum_ f(p^k) = f(2) + f(3) + f(4) +f(5) +f(7)+f(8)+f(9)+\cdots
\sum_ f(d)\;   and   \prod_ f(d)\;   mean that the sum or product is over all positive divisors of ''n'', including 1 and ''n''. E.g., if ''n'' = 12,
:\prod_ f(d) = f(1)f(2) f(3) f(4) f(6) f(12).\
The notations can be combined:   \sum_ f(p)\;   and   \prod_ f(p)\;   mean that the sum or product is over all prime divisors of ''n''. E.g., if ''n'' = 18,
:\sum_ f(p) = f(2) + f(3),\
and similarly   \sum_ f(p^k)\;   and   \prod_ f(p^k)\;   mean that the sum or product is over all prime powers dividing ''n''. E.g., if ''n'' = 24,
:\prod_ f(p^k) = f(2) f(3) f(4) f(8).\

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Arithmetic function」の詳細全文を読む



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