Khayyam was famous during his life as a
mathematician. His surviving mathematical works include • (i) ''Commentary on the Difficulties Concerning the Postulates of Euclid's Elements'' (), completed in December 1077, • (ii)
Treatise On the Division of a Quadrant of a Circle (), undated but completed prior to the
Treatise on Algebra, Drawing upon
Aristotle's views, he rejects the usage of movement in geometry and therefore dismisses the different attempt by
Ibn al-Haytham. Unsatisfied with the failure of mathematicians to prove Euclid's statement from his other postulates, Khayyam tried to connect the axiom with the Fourth Postulate, which states that all right angles are equal to one another. His elaborate attempt to prove the parallel postulate was significant for the further development of geometry, as it clearly shows the possibility of non-Euclidean geometries. The hypotheses of acute, obtuse, and right angles are now known to lead respectively to the non-Euclidean
hyperbolic geometry of Gauss-Bolyai-Lobachevsky, to that of
elliptic geometry, and to
Euclidean geometry.
Tusi's commentaries on Khayyam's treatment of parallels made their way to Europe.
John Wallis, professor of geometry at
Oxford, translated Tusi's commentary into Latin. Jesuit geometer
Girolamo Saccheri, whose work (
euclides ab omni naevo vindicatus, 1733) is generally considered the first step in the eventual development of
non-Euclidean geometry, was familiar with the work of Wallis. The American historian of mathematics
David Eugene Smith mentions that Saccheri "used the same lemma as the one of Tusi, even lettering the figure in precisely the same way and using the lemma for the same purpose". He further says that "Tusi distinctly states that it is due to Omar Khayyam, and from the text, it seems clear that the latter was his inspirer."
Real number concept This treatise on Euclid contains another contribution dealing with the
theory of proportions and with the compounding of ratios. Khayyam discusses the relationship between the concept of ratio and the concept of number and explicitly raises various theoretical difficulties. In particular, he contributes to the theoretical study of the concept of
irrational number. Displeased with Euclid's definition of equal ratios, he redefined the concept of a number by the use of a continued fraction as the means of expressing a ratio.
Youschkevitch and Rosenfeld argue that "by placing irrational quantities and numbers on the same operational scale, [Khayyam] began a true revolution in the doctrine of number." In the
Treatise on the Division of a Quadrant of a Circle Khayyam applied algebra to geometry. In this work, he devoted himself mainly to investigating whether it is possible to divide a circular quadrant into two parts such that the line segments projected from the dividing point to the
perpendicular diameters of the circle form a specific ratio. His solution, in turn, employed several curve constructions that led to equations containing cubic and quadratic terms. The
Treatise on Algebra contains his work on
cubic equations. It is divided into three parts: (i) equations which can be solved with
compass and straight edge, (ii) equations which can be solved by means of
conic sections, and (iii) equations which involve the
inverse of the unknown. He considered three binomial equations, nine trinomial equations, and seven tetranomial equations. For these he could not accomplish the construction of his unknown segment with compass and straight edge. He proceeded to present geometric solutions to all types of cubic equations using the properties of conic sections. The prerequisite lemmas for Khayyam's geometrical proof include
Euclid VI, Prop 13, and
Apollonius II, Prop 12. However, he acknowledged that the arithmetic problem of these cubics was still unsolved, adding that "possibly someone else will come to know it after us". This particular geometric solution of cubic equations was further investigated by
M. Hachtroudi and extended to solving fourth-degree equations. Although similar methods had appeared sporadically since
Menaechmus, and further developed by the 10th-century mathematician
Abu al-Jud, Khayyam's work can be considered the first systematic study and the first exact method of solving cubic equations. The mathematician
Woepcke (1851) who offered translations of Khayyam's algebra into French praised him for his "power of generalization and his rigorously systematic procedure."
Binomial theorem and extraction of roots In his algebraic treatise, Khayyam alludes to a book he had written on the extraction of the nth root of natural numbers using a law he had discovered which did not depend on geometric figures. One of Khayyam's predecessors,
al-Karaji, had already discovered the triangular arrangement of the coefficients of binomial expansions that Europeans later came to know as
Pascal's triangle; Khayyam popularized this
triangular array in Iran, so that it is now known as Omar Khayyam's triangle. ==Astronomy==