>>64
Modern Computer Arithmetic
Richard P. Brent and Paul Zimmermann
Version 0.5.9 of 7 October 2010
http://www.loria.fr/~zimmerma/mca/mca-cup-0.5.9.pdf

上記のテクストの
4.8.2 First AGM algorithm for the logarithm
4.8.4 Second AGM algorithm for the logarithm
とかご参考まで.