Editing Logarithm (section)

===Arithmetic–geometric mean approximation===
The [[arithmetic–geometric mean]] yields high-precision approximations of the [[natural logarithm]]. Sasaki and Kanada showed in 1982 that it was particularly fast for precisions between 400 and 1000 decimal places, while Taylor series methods were typically faster when less precision was needed. In their work {{math|ln(''x'')}} is approximated to a precision of {{math|2<sup>−''p''</sup>}} (or {{Mvar|p}}&nbsp;precise bits) by the following formula (due to [[Carl Friedrich Gauss]]):<ref>{{Citation |first1=T. |last1=Sasaki |first2=Y. |last2=Kanada |title=Practically fast multiple-precision evaluation of log(x) |journal=Journal of Information Processing |volume=5|issue=4 |pages=247–50 |year=1982 | url=http://ci.nii.ac.jp/naid/110002673332 | access-date=30 March 2011}}</ref><ref>{{Citation |first1=Timm |title=Stacs 99|last1=Ahrendt|publisher=Springer|location=Berlin, New York|series=Lecture notes in computer science|doi=10.1007/3-540-49116-3_28|volume=1564|year=1999|pages=302–12|isbn=978-3-540-65691-3|chapter=Fast Computations of the Exponential Function}}</ref>

<math display="block">\ln (x) \approx \frac{\pi}{2\, \mathrm{M}\!\left(1, 2^{2 - m}/x \right)} - m \ln(2).</math>

Here {{math|M(''x'', ''y'')}} denotes the [[arithmetic–geometric mean]] of {{mvar|x}} and {{mvar|y}}. It is obtained by repeatedly calculating the average {{Math|(''x'' + ''y'')/2}} ([[arithmetic mean]]) and <math display="inline">\sqrt{xy}</math> ([[geometric mean]]) of {{mvar|x}} and {{mvar|y}} then let those two numbers become the next {{mvar|x}} and {{mvar|y}}. The two numbers quickly converge to a common limit which is the value of {{math|M(''x'', ''y'')}}. {{mvar|m}} is chosen such that

<math display="block">x \,2^m > 2^{p/2}.\, </math>

to ensure the required precision. A larger {{mvar|m}} makes the {{math|M(''x'', ''y'')}} calculation take more steps (the initial {{mvar|x}} and {{mvar|y}} are farther apart so it takes more steps to converge) but gives more precision. The constants {{math|{{pi}}}} and {{math|ln(2)}} can be calculated with quickly converging series.