Khayyam, Omar vi. As mathematician. B. Vahabzadeh.
Encyclopaedia Iranica Online, May 2014.

Three mathematical treatises of Omar Khayyam have come down to us: (1) a commentary on Euclid’s Elements; (2) an essay on the division of the quadrant of a circle; (3) a treatise on algebra; and (4) the treatise on the extraction of the nth root of the numbers, which is not extant.