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Arithmetic–geometric mean
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Arithmetic–geometric mean : ウィキペディア英語版
Arithmetic–geometric mean

In mathematics, the arithmetic–geometric mean (AGM) of two positive real numbers and is defined as follows:
First compute the arithmetic mean of and and call it . Next compute the geometric mean of and and call it ; this is the square root of the product :
:\begin
a_1 &= \tfrac12(x + y)\\
g_1 &= \sqrt
\end
Then iterate this operation with taking the place of and taking the place of . In this way, two sequences and are defined:
:\begin
a_ &= \tfrac12(a_n + g_n)\\
g_ &= \sqrt
\end
These two sequences converge to the same number, which is the arithmetic–geometric mean of and ; it is denoted by , or sometimes by .
This can be used for algorithmic purposes as in the AGM method.
==Example==
To find the arithmetic–geometric mean of and , first calculate their arithmetic mean and geometric mean, thus:
:\begin
a_1 &= \tfrac12(24 + 6) = 15\\
g_1 &= \sqrt = 12
\end
and then iterate as follows:
:\begin
a_2 &= \tfrac12(15 + 12) = 13.5\\
g_2 &= \sqrt = 13.41640786500\dots\\
\dots
\end
The first five iterations give the following values:
:
As can be seen, the number of digits in agreement (underlined) approximately doubles with each iteration. The arithmetic–geometric mean of 24 and 6 is the common limit of these two sequences, which is approximately 13.4581714817256154207668131569743992430538388544.〔(agm(24, 6) at WolframAlpha )〕

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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