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Irrationality sequence : ウィキペディア英語版
Irrationality sequence
In mathematics, a sequence of positive integers ''a''''n'' is called an irrationality sequence if it has the property that, for every sequence ''x''''n'' of positive integers, the sum of the series
: \sum_^\infty \frac
exists (that is, it converges) and is an irrational number.〔.〕〔.〕 The problem of characterizing irrationality sequences was posed by Paul Erdős and Ernst G. Straus, who originally called the property of being an irrationality sequence "Property P".〔.〕
==Examples==
The powers of two whose exponents are powers of two, 2^, form an irrationality sequence. However, although Sylvester's sequence
:2, 3, 7, 43, 1807, 3263443, ...
(in which each term is one more than the product of all previous terms) also grows doubly exponentially, it does not form an irrationality sequence. For, letting x_n=1 gives
:\frac+\frac+\frac+\frac+\cdots=1,
a series converging to a rational number. Likewise, the factorials n! do not form an irrationality sequence, because the sequence x_n=n+2 leads to a series with a rational sum,
:\sum_^\frac=\frac+\frac+\frac+\frac+\frac+\cdots=1.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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