Sangaku: Reflections on the Phenomenon
Year after year On the monkey's face A monkey's mask. 
Matsuo Basho (16441694) 
Sangaku are often colorful tablets offered in shinto shrines (and sometimes in buddhist temples) in Japan and posing mathematical problems. The earliest sangaku date a few years before the beginning of the japanese Edo period (16031867) of selfimposed seclusion from the Western world. Most of the writeups on the Sangaku phenomenon are based on either a Scientific American article by Tony Rothman written in cooperation with Hidetoshi Fukagawa, a Japanese teacher with a Ph.D in mathematics, or the book by H. Fukagawa and D. Pedoe. For example, Rothman explains in the introduction to his article:
Of the world's countless customs and traditions, perhaps none is as elegant, nor as beautiful, as the tradition of sangaku, Japanese temple geometry. From 1639 to 1854, Japan lived in strict, selfimposed isolation from the West. Access to all forms of occidental culture was suppressed, and the influx of Western scientific ideas was effectively curtailed. During this period of seclusion, a kind of native mathematics flourished.
Devotees of math, evidently samurai, merchants and farmers, would solve a wide variety of geometry problems, inscribe their efforts in delicately colored wooden tablets and hang the works under the roofs of religious buildings. These sangaku, a word that literally means mathematical tablet, may have been acts of homagea thanks to a guiding spiritor they may have been brazen challenges to other worshipers: Solve this one if you can! For the most part, sangaku deal with ordinary Euclidean geometry. But the problems are strikingly different from those found in a typical high school geometry course. Circles and ellipses play a far more prominent role than in Western problems: circles within ellipses, ellipses within circles. Some of the exercises are quite simple and could be solved by firstyear students. Others are nearly impossible, and modern geometers invariably tackle them with advanced methods, including calculus and affine transformations.
Introducing a Sangoku problem, the authors of a remarkable problem collection Which Way Did the Bicycle Go? quote from the book by H. Fukagawa and D. Pedoe:
During the greater part of the Edo period (16031867) Japan was almost completely cut off from the western world. Books on mathematics, if they entered Japan at all, must have been scarce, and yet, during this long period of isolation people of all social classes, from farmers to samurai, produced theorems in Euclidean geometry which are remarkably different from those produced in the west during the centuries of schism, and sometimes anticipated these theorems by many years.
These theorems were not published in books, but appeared as beautifully coloured drawings on wooden tablets which were hung under the roof in the precincts of a shrine or temple.
Remarkably, where Rothman notes the difference between Sangaku and school geometry problems, Pedoe contrasts Sangaku with the geometry "produced in the west". In general, I believe, the tendency to exaggerate the significance of the Sangaku phenomenon grows with the followup writings. For example, Chad Boutin starts his paper in News@Princeton thus
Before I proceed, I wish to assure the reader of my deep appreciation of the quality and beauty of many Sangaku problems. At the bottom of this page there is a list of problems discussed at this site. I just question the plausibility of the historic picture that emerges from the above quotes. Essentially, I doubt that the practice was widely spread or that it was much affected by the atmosphere of seclusion. It perhaps relevant to note that during this same period Haiku became a national activity. The commonly given reason for the growth of Haiku's stature in Japanese culture is the involvement of two masters (Matsuo Basho and Onitsura) who elevated haiku to new artistic heights. Never did I see the ascendency of Haiku related to the particulars of the period, although, in all likelihood, it was. By limiting the influx of intellectual stimuli, Sakoku, as this period of selfimposed seclusion is known in Japanese, may have caused a more focused concentration on the homegrown developments.
First of all, Sangaku problems were written in Kanbun, Chinese written for the Japanese audience. While Japan may boast high level of literacy even during the Edo period, Kanbun is said to be the Japanese equivalent of Latin, with the obvious implication. As [Fukagawa & Pedoe, Preface] note, ... Kanbun ... can be read by only a small number of people in modern Japan. [Fukagawa and Rothman, p. 9] note:
This makes it hard to believe that Kanbun was widely known during the Edo period, either.
Second, Sakoku lasted a little more than 200 years, from 1647 through 1854. (The sangaku tradition has started earlier and ended later so that I estimate its duration at 250 years.) Europe, during the first one hundred years of Sakoku, saw the emergence of Calculus but not much of geometry, except, perhaps for a few Euler's results. In Europe, in the second half of Sakoku, numerous cases have been documented (e.g., Wessel's contribution to complex numbers) when important mathematical results have been overlooked by the mathematical community or have been independently rediscovered in different corners of the continent. So waxing sentimental over "the centuries of schism" is not quite justified. If anything, proliferation of Sangaku demonstrates the effectiveness of popularization. Reaching the broad masses via beautiful and mysterious wooden tablets displayed at places of worship and congregation, mathematics got a foothold in Japanese culture. But even this did not last long. By the 20^{th} century, the Sangaku tradition was all but forgotten.
In her biography of H. M. S. Coxeter, Siobhan Roberts quotes her correspondence (20032005) with Koji Miyazaki, the author of 35 books on various aspects of geometry. "Us Japanese originally don't like geometric logic and if there are no Coxeter's understandable geometry us Japanese have not become aware of morphologic contents about things. Japanese version of his book 'Introduction into Geometry' gave much impact to so many Japanese including me."
Third, it is also said that
I read from here that the Japanese did not lose much, at least not for the first 100 years of seclusion.
Fourth, another problem with the quotes is that during the Edo period, the population was divided into four classes: the samurai on top (about 5% of the population), and the peasants (more than 80% of the population) on the second level. Below the peasants were the craftsmen, and even below them, on the fourth level, were the merchants. So, perhaps, the expression "... people of all social classes, from farmers to samurai, produced theorems ..." is not quite accurate and does not probably express correctly the authors intention to hint at the broad popularity of Sangaku. It certainly does not embrace all the strata of the contemporary Japanese society.
Fifth, by the mideighteenth century, population of Edo  the future Tokyo  reached 1,000,000. That of Kyoto and Osaka was around 400,000 each. On the other hand, the highest number of surviving Sangaku I came across is given as 880. On some the problems are barely visible. If one did not know better, they could be mistaken for the plain wooden pieces. I think an estimate of a total of 5000 produced during the period of seclusion may be quite plausible if not exaggerated. It appears that, on average, during 250 years of Sakoku about 20 Sangaku have been created per year. Which looks as a possible outcome of a small group of disciples rather than the broad population as seen implied by the quote ("people of all social classes".)
A later correction: Chapter 7 of the new book by Fukagawa and Rothman narrates the story of Kazan Yamaguchi, a Japanese mathematician who undertook six "sangaku pilgrimages" visiting the temples around the country over the period between 1817 and 1828. Yamaguchi's diary that runs about 700 pages documents 87 sangaku of which only 2 have survived to the present. Using the survival factor of 87/2 the above calculations lead to
Sixth, many of the surviving Sangaku come in groups of related problems, see for example, Sequences of Touching Circles, Sangakus with a Mixtilinear Circle or Circles in a Circular Segment. ([Fukagawa & Pedoe, Preface] also suggest that, towards the end of seclusion, there was undoubted plagiarism.) Related problems appeared in different prefectures, not just in different shrines. Which serves to indicate that much of the Sangaku proliferation was due to travel, either by a peripatetic school or wandering individuals.
The foregoing thoughts should not be construed as the lack of appreciation of the Sangaku phenomenon or of particular problems. Many of the problems I have seen have charming elegance. I just do not feel they need any kind of extraneous mystification. They deserve full appreciation simply for what they are.
In a 1914 book, D. E. Smith and Y. Mikami mention temple geometry in a more level headed manner (p. 184):
(Noteworthy: Jean Constant, a contemporary artist, has been inspired by the tablets to create many wonderful artwork he collected in several online galleries.)
References
 H. Fukagawa, D. Pedoe, Japanese Temple Geometry Problems, The Charles Babbage Research Center, Winnipeg, 1989
Write to:
Charles Babbage Research Center
P.O. Box 272, St. Norbert Postal Station
Winnipeg, MB
Canada R3V 1L6  H. Fukagawa, A. Rothman, Sacred Mathematics: Japanese Temple Geometry, Princeton University Press, 2008
 J. Konhauser, D. Velleman, S. Wagon, Which Way Did the Bicycle Go?, MAA, 1996, #50
 S. Roberts, King of Infinite Space, Walker & Company, 2006
 D. E. Smith and Yoshio Mikami, A History of Japanese Mathematics, Dover, 2004 (originally 1914)
Sangaku

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