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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 : 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, : : For example, : : ==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, : has a period of 18. Dividing the repeating portion into 6-digit numbers and summing them gives : Similarly, dividing the repeating portion into 3-digit numbers and summing them gives : ==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 : In duodecimal (using inverted two and three for ten and eleven, respectively) : 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 スポンサード リンク
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