Wada Yenzō Nei (和田 寧, 1787 – October 13, 1840), also known as Wada Yasushi, was a Japanese mathematician in the Edo period. His birth name was Kōyama Naoaki; but he changed his name to Wada Nei, by which he became more widely known.^[1]

Wada studied under the patronage of the hereditary Chief court calendar-maker (暦博士, Reki-hakase)^[2] in the court of the emperor.^[3]

Wada became a student of Kusaka Sei, who had been a student of Ajima Chokuyen. Wada extended Ajima Chokuyen's development of an integral calculus within the Enri (円理, "circle principle") context. He worked on the computation of minimum and maximum values (roughly by equating the first derivative to 0) and gave reasoning and insight to the computation method that was given without explanation by Seki Takakazu about 100 years earlier. He was also the first Japanese mathematician to study roulettes.

Wada's published writings are few.^[4]

• Enri kigen (員理起元) OCLC 22035715224

• Sangaku, the custom of presenting mathematical problems, carved in wood tablets, to the public in shinto shrines
• Soroban, a Japanese abacus
• Japanese mathematics

• Endō Toshisada (1896). History of mathematics in Japan (日本數學史史, Dai Nihon sūgakush). Tōkyō: _____. OCLC 122770600
• Restivo, Sal P. (1992). Mathematics in Society and History: Sociological Inquiries. Dordrecht: Kluwer Academic Publishers. ISBN 9780792317654; OCLC 25709270
• David Eugene Smith and Yoshio Mikami. (1914). A History of Japanese Mathematics. Chicago: Open Court Publishing. OCLC 1515528 -- note alternate online, full-text copy at archive.org
• Isaac Titsingh. (1834). Nihon Odai Ichiran; ou, Annales des empereurs du Japon. Paris: Royal Asiatic Society, Oriental Translation Fund of Great Britain and Ireland. OCLC 5850691