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Hermite's problem is an open problem in mathematics posed by Charles Hermite in 1848. He asked for a way of expressing real numbers as sequences of natural numbers, such that the sequence is eventually periodic precisely when the original number is a cubic irrational. ==Motivation== A standard way of writing real numbers is by their decimal representation, such as: : where ''a''0 is an integer, the integer part of ''x'', and ''a''1, ''a''2, ''a''3… are integers between 0 and 9. Given this representation the number ''x'' is equal to : The real number ''x'' is a rational number only if its decimal expansion is eventually periodic, that is if there are natural numbers ''N'' and ''p'' such that for every ''n'' ≥ ''N'' it is the case that ''a''''n''+''p'' = ''a''''n''. Another way of expressing numbers is to write them as continued fractions, as in: : where ''a''0 is an integer and ''a''1, ''a''2, ''a''3… are natural numbers. From this representation we can recover ''x'' since : If ''x'' is a rational number then the sequence (''a''''n'') terminates after finitely many terms. On the other hand, Euler proved that irrational numbers require an infinite sequence to express them as continued fractions.〔(【引用サイトリンク】url=http://math.dartmouth.edu/~euler/pages/E101.html )〕 Moreover, this sequence is eventually periodic (again, so that there are natural numbers ''N'' and ''p'' such that for every ''n'' ≥ ''N'' we have ''a''''n''+''p'' = ''a''''n''), if and only if ''x'' is a quadratic irrational. 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「Hermite's problem」の詳細全文を読む スポンサード リンク
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