Li Zhi (mathematician)

Li Ye (Chinese: 李冶; Wade–Giles: Li Yeh; 1192–1279), born Li Zhi (Chinese: 李治), was a Chinese mathematician and scholar, who published and improved the tian yuan shu method for solving polynomial equations of one variable.

He was born Li Zhi, but later changed his name to Li Ye to avoid confusion with the third Tang emperor who was also named Li Zhi, removing one stroke from his original name to change the character. His name is also sometimes written as Li Chih or Li Yeh. The literary name was 仁卿 (Renqing, Jen-ch’ing) and appellation was 敬斋 (Jingzhai, Ching-chai).

Li Ye was born in Daxing (now Beijing). His father was a secretary to an officer in the Jurchen army. Li passed the civil service examination in 1230 at the age of 38, and was administrative prefect of Jun prefecture in Henan province until the Mongol invasion in 1233. He then lived in poverty in the mountainous Shanxi province. In 1248 he finished his most known work Ceyuan haijing (測圓海鏡, Sea mirror of circle measurements). Li then returned to Hebei.

In 1257 Kublai Khan, grandson of Genghis Khan, sent for Li to get advice on government and science. In 1259 Li completed Yigu yanduan (益古演段, New steps in computation), also an important mathematics text. After becoming Khan, Kublai twice offered Li government positions. He politely declined, referring to his age and ill health. In 1264 Li finally accepted a position at the Hanlin Academy, writing official histories. He was critical of the politics and resigned after a few months, again citing ill health. He spent his final years teaching at his home near Feng Lung mountain in Yuan, Hebei. Li told his son to burn all of his books except for Sea mirror of circle measurements. Fortunately other texts, both mathematical and literary, still survived.

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