Madhava's Pi-Series in the Modern Context

Abstract

The $π$-series is attributed to Madhava, Gregory and Leibniz based on the chronology of its discovery. While this acknowledges the fact that Madhava of Sangamagrama an Indian mathematician who lived in Kerala in the 14th century along the banks of river Nila, was the first to discover the series, the modern day proof for the $π$-series is based on the outline of the proof given by the famous German mathematician Gottfried Wilhelm Leibniz in the 17th century, using concepts and rigorous definitions of Calculus many of which were developed after his lifetime. We extend the same benefit to Madhava's ideas and present Madhava's upapatti i.e proof for the $π$-series as given in Jyesthadeva's Yuktibhasha using modern notation and definitions. We elaborate Madhava's proof at three places (a) to prove that the arc-bit sum is same as the sum of arc-bit approximations (b) to prove convergence and (c) to concisely prove by iteration the power sum approximation. Madhava's method with appropriate changes can be applied to derive the sine and cosine series as well. Madhava also provides the correction terms to accelerate the convergence of the $π$-series. We compare the two proofs i.e Madhava's and the popular proof for $π$ which is based on Leibniz's ideas and observe that they are different since Madhava's is based on the length of the arc where as Leibniz's relies on the area under the arc and in terms of the number of concepts from advanced calculus used in the the two proofs Madhava's proof stands out for its simplicity, thus making a case for it to be appreciated, popularised and taught as the original proof of the $π$-series.