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Midy's theorem : ウィキペディア英語版
Midy's theorem
In mathematics, Midy's theorem, named after French mathematician E. Midy, is a statement about the decimal expansion of fractions ''a''/''p'' where ''p'' is a prime and ''a''/''p'' has a repeating decimal expansion with an even period . If the period of the decimal representation of ''a''/''p'' is 2''n'', so that
:\frac=0.\overline}
then the digits in the second half of the repeating decimal period are the 9s complement of the corresponding digits in its first half. In other words,
:a_i+a_=9 \,
:a_1\dots a_n+a_\dots a_=10^n-1. \,
For example,
:\frac=0.\overline\text076+923=999. \,
:\frac=0.\overline\text05882352+94117647=99999999. \,
==Extended Midy's theorem==
If ''k'' is any divisor of the period of the decimal expansion of ''a''/''p'' (where ''p'' is again a prime), then Midy's theorem can be generalised as follows. The extended Midy's theorem〔Bassam Abdul-Baki, (''Extended Midy's Theorem'' ), 2005.〕 states that if the repeating portion of the decimal expansion of ''a''/''p'' is divided into ''k''-digit numbers, then their sum is a multiple of 10''k'' − 1.
For example,
:\frac=0.\overline \,
has a period of 18. Dividing the repeating portion into 6-digit numbers and summing them gives
:052631+578947+368421=999999.
Similarly, dividing the repeating portion into 3-digit numbers and summing them gives
:052+631+578+947+368+421=2997=3\times999.
==Midy's theorem in other bases==
Midy's theorem and its extension do not depend on special properties of the decimal expansion, but work equally well in any base ''b'', provided we replace 10''k'' − 1 with ''b''''k'' − 1 and carry out addition in base ''b''.
For example, in octal
:
\begin
& \frac=0.\overline_8 \\()
& 032_8+745_8=777_8 \\()
& 03_8+27_8+45_8=77_8.
\end

In duodecimal (using inverted two and three for ten and eleven, respectively)
:
\begin
& \frac=0.\overline_ \\()
& 076_+\mathcal45_=\mathcal_ \\()
& 07_+6\mathcal_+45_=\mathcal_
\end


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「Midy's theorem」の詳細全文を読む



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