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In mathematics, a Somos sequence is a sequence of numbers defined by a certain recurrence relation, described below. They were discovered by mathematician Michael Somos. It is not obvious from the form of their defining recurrence that every number in a Somos sequence is an integer, but nevertheless many Somos sequences have the property that all of their members are integers. ==Recurrence equations== For an integer number ''k'' larger than 1, the Somos-''k'' sequence is defined by the equation : when ''k'' is odd, or by the analogous equation : when ''k'' is even, together with the initial values : ''a''''i'' = 1 for ''i'' < ''k''. For ''k'' = 2 or 3, these recursions are very simple (there is no addition on the right-hand side) and they define the all-ones sequence (1, 1, 1, 1, 1, 1, ...). In the first nontrivial case, ''k'' = 4, the defining equation is : while for ''k'' = 5 the equation is : These equations can be rearranged into the form of a recurrence relation, in which the value ''a''''n'' on the left hand side of the recurrence is defined by a formula on the right hand side, by dividing the formula by ''a''''n'' − ''k''. For ''k'' = 4, this yields the recurrence : While in the usual definition of the Somos sequences, the values of ''a''''i'' for ''i'' < ''k'' are all set equal to 1, it is also possible to define other sequences by using the same recurrences with different initial values. 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「Somos sequence」の詳細全文を読む スポンサード リンク
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