On the computation of the arcsin function in the Kerala school of astronomy and mathematics

Abstract

This paper examines how the mathematicians and astronomers of the Kerala school tackled the problem of computing the values of the arcsin function. Four different approaches are discussed all of which are found in Nilakantha Somayaji's (1444 - 1545 CE) Tantrasangraha and the roots of all of which can be traced to ideas originally articulated by Sangamagrama Madhava (c. 1340 - 1425 CE): (i) a simple method when the argument is small; (ii) an iterative method when the argument is small; (iii) a method based on a lookup table; (iv) a method when the argument is large. The paper also contains the original Sanskrit verses describing the various methods and English translations thereof. Moreover, there is a presentation of a novel method for computing the circumference of a circle found in Jyeshthadeva's (c. 1500 - 1575 CE) Yuktibhasha which is based on method (i) for computing the arcsin function. All methods have been illustrated with numerical examples. A surprising by-product of the investigation is a totally unexpected appearance of a core integer sequence, namely, the entry A001764 in the Online Encyclopedia of Integer Sequence, while studying the iterative method for computing the arcsin function.