אברהם בר חייא

Rav Avraham bar Chiyya HaNasi (c. 4830/1070–1136), known in Latin sources as Abraham Savasorda and Abraham Judaeus, was a rishon of Barcelona and one of the foremost Jewish mathematicians, astronomers, and thinkers of his age. He carried the title Nasi and, in Arabic, the office of commander of the guard, and held a high place at court. According to a tradition brought in his name, he was a descendant of Chizkiyahu, the last Gaon of the academies of Bavel.

His labors placed him among the great intermediaries who brought the science of the Arabic world to Christian Europe, both through his own sefarim and through his translations — he worked alongside the Latin translator Plato of Tivoli. His Chibbur haMeshichah vehaTishboret, a Hebrew treatise on measurement and calculation rendered into Latin as Liber embadorum, contains the first known full solution of the general quadratic equation, and it became one of the sources on which Fibonacci built his own work on practical geometry. His encyclopedic Yesodei haTevunah uMigdal haEmunah was an early European attempt to draw together the mathematics of the Greeks and the Arabs.

He wrote all his works in Hebrew rather than in the Judeo-Arabic of the earlier Sephardic scholars, coining much of the scientific and philosophical vocabulary and standing among the founders of scientific writing in the language. His Sefer haIbbur, on the calculation of the Hebrew calendar, is the oldest known Hebrew work of its kind, and the Rambam praised it. In matters of faith he wrote Hegyon haNefesh, an ethical work on teshuvah built around the haftarot of Yom Kippur and Shabbat Shuvah, and Megillat haMegalleh on the redemption of Klal Yisrael, in which he reckoned the coming of Mashiach for the year 5118/1358. Later rishonim including the Ibn Ezra and the Radak cite from his sefarim.