Sakabe Kōhan

Who was Sakabe Kōhan?

Sakabe Kōhan was a Japanese mathematician in the Edo period.

Sakabe served for a time in the Fire Department of the shogunate, but he resigned that position to become a ronin or masterless samurai. He spent the rest of this life in study, in teaching, and in promoting mathematics education in Japan.

Sakabe was a student of Ajima Naonobu.

Sakabe investigated some European and Chinese works which had appeared in Japan, but his general a method was later construed to be innovative, clarified and thus improved. Foreign influence shows itself indirectly some of his published work.

Sakabe's Sampo Tenzan Shinan-roku in 1810 was the first published work in Japan proposing the use of logarithmic tables. He explained that "these tables save much labor, [but] they are but little known for the reason that they have never been printed in our country." Sakabe's proposal would not be realized until twenty years after his death when the first extensive logarithmic table was published in 1844 by Koide Shuki.

In Sakabe's Treatise on Tenzan Algebra, mathematical problems are arranged in order from easy problems to difficult ones. The text presents a method for finding the length of a circumference and the length an arc of an ellipse. This was the first appearance of the problems pertaining to ellipses in printed books in Japan.