It was only in the twentieth century that certain Indian mathematicians brought it to the attention of a more general audience. Even then it remained under the esoteric preserve of academics and researchers of mathematical history and has still not become mainstream knowledge.
The lineage of Madhava and his disciples (Guru-Shishya parampara) and their works
Starting from the 13th century, a lineage of Sanskrit scholars in Kerala specializing in astronomy, mathematics, and linguistics developed the basic ideas of calculus, solutions of transcendental equations by iteration, infinite series expansions for the sine, cosine and inverse tangent functions, infinite series analysis with tests of convergence of infinite series, approximation of transcendental numbers by continued fractions, etc. . These new advances were initiated by Madhava of Sangamagrama and his disciples. He and his disciples formed an unbroken lineage and knowledge was transmitted in the classic Guru-shishya parampara (teacher-disciple tradition; in Sanskrit guru = teacher, shishya = disciple, parampara = tradition). Madhava's lineage is today known as the Kerala school of mathematics in academic circles.
Some historical facts first. Madhava of Sangamagrama was born in the town of Sangamagrama. This is the present day town of Irinjalakuda in Thrissur district of Kerala (shown in red in the map). It is quite close to Kaladi, birth place of Adi Shankara (shown in blue in the map). Madhava and his disciples were Nambudiri Brahmins and were active in the temple of Guruvayoor, which is a temple dedicated to Lord Sri Krishna. Most of the astronomers and mathematicians of the Madhava tradition were worshippers of Sri Krishna, and even today the temple is one of the holiest spots of pilgrimage for Hindus and is known as the Dwaraka of the south.
Mathematical highlights of the Madahava school
Now we come to the mathematical work of the Kerala scholars.
As discussed in the earlier posts, Indian mathematicians and scholars writing in Sanskrit such as Aryabhata, Bhaskara, Brahmagupta, etc. laid the foundations of astronomy, algebra, arithmetic, trigonometry, linguistics in their seminal works (for more information read the earlier posts on this blog). The Kerala mathematicians carried forward the work of these scholars to new levels of sophistication. Madhava's results predate Europe by more than 3 centuries, where these results are today (wrongly) believed to have been originally discovered. Here I shall give a very short summary of some of the contributions of the Kerala mathematicians pertaining to calculus. I will ignore their works in linguistics and astronomy since it is beyond the scope of this blog post.
The towering figure of the Madhava school is of course Madhava. Very few of his works survive in original form. He is known to have written the texts Venvaroha, Sphutachandrapti, Aganitagrahachara, amongst others. His disciples have stated and elaborated several of his results in their own works. One of Madhava's most striking results concerns the series expansions of the sine and cosine functions which has been referred to, for example, by Nilakantha Somayaji (1445-1545) in his Aryabhatia Bhashya of the 13th-14th century (pg. 155 of Ref. ) These series are well known:
The importance of these series goes far beyond these series themselves - their importance lies in the fact that it opens a whole new doorway into the analysis of infinitesimals and limiting quantities, which forms the foundation of differential and integral calculus and the branch of mathematical analysis and the study of infinite series and their convergence. In fact Madhava "took the decisive step onward from the finite procedures of ancient mathematics to treat their limit passage to infinity, which is the kernel of modern classical analysis."  This doorway was opened in Europe a full four centuries after Madhava composed his works, and as I will discuss later, it was not perhaps an entirely independent development.
Here we have a page from Nilakantha's Aryabhatia Bhashya (see ):
A closely related discovery by Madhava is the series for the inverse tangent function, which in modern terminology is expressed as:
Madhava has been credited for this discovery, for example, by Nilakantha Somayaji (1445-1545) in his Tantrasangraha ( and ) and by Shankara Varman (1800-1838) in Sadratnamala. A proper proof of this result is also given in Jyeshthadeva's (1500-1610) Yuktibhasha. The verse in Sanskrit from the Sadratnamala is shown in the following figure
The tranlsation is: "The radius into the sine divided by the cosine is the first quote: this multiplied by the square of the sine, and divided by the square of the cosine, is the second quote; this second, and those obtained continually in the same way, multiply and divide by the square of the sine and the square of the cosine respectively: divide the quotes in order by 1, 3, 5, 7, 11, etc. respectively, and the difference of the sum of the 1st, 3rd,5th, etc. and of the 2nd, 4th, 6th, etc., will be the arc whose sine was taken.'' Verse taken from Ref. , translation from . The Sanskrit words and their meanings as far as I know (thanks to ). phala - result / area; guna - multiple; bhuja/bahu - side (of a triangle/swaure etc.); dhanus - arc of a circle; varga - square; koti - height; yugma - even.
Irrationality of π
Already in the 5th century, i.e. 800 years before the Kerala mathematicians, Aryabhata was well aware of the irrationality of the ratio of circumference to diameter of a circle (π). In the following verse in the Aryabhatia,
The possible transmission of the Kerala mathematics to Europe
Although the development of calculus is conventionally attributed to Newton and Leibniz, it is quite possible that its development by these two mathematicians was not unassisted and independent of the Kerala mathematics. In a separate I will discuss this more detail. Here I will just give an overview of the circumstantial evidence which supports this possibility. The information given below is from a slide show I created earlier. (Here is a link to the slide show.)
During the middle ages, Europe was centuries behind India in mathematical knowledge. It was only in 1202 that the Indian number system was popularized in Europe by Fibonacci through the Arabs (hence the number system is mistakenly called the Arabic number system). This was the time when Europe was engulfed in the Dark Ages and abject poverty, hence trade and conquest of wealthy nations such as India assumed paramount importance. But trade implies navigating the seas, which needs knowledge of astronomy, for e.g. calculating latitude and longitude, which in turn requires knowledge of trigonometry, tables of sines, cosines, etc. A reliable calendar is also a must for these purposes. Now neither did Europeans have this knowledge nor was their calendar reliable enough for navigation, which resulted in the loss of several ships as well as severe economic and human losses. Since the church dominated the scene in Europe, navigation and calendar reform became priority programs by the church. Lucrative prizes were offered by the church to anyone who could provide accurate navigational techniques and calendrical reforms.
At the same time, the Hindus in India had all this information in the form of the Sanskrit texts of Bhaskara, Aryabhata, etc. Indian navigators, assisted by reliable navigation techniques and an accurate calendar, also used to trade with several countries. It became important for the church to obtain this information from the Hindus. But Hindus were 'pagans', 'heathens' and 'idol worshippers' who had to be 'civilized' (christianized). It would never do for the church to admit that the 'pagan' Hindus had superior knowledge. Hence although it privately sought 'pagan' learning, publicly the church continued to deny that there was any learning among the 'pagans'. There was great pressure among the people to deny the possibility of the pagans possessing useful knowledge, since death in the form of being burnt at the stake for being a 'heretic' would be a certainty for such a person.
The opportunity and means for obtaining the knowledge appeared from 1499 onwards. In this year, Vasco da Gama arrived at the Malabar coast in Kerala and established a direct link to Europe via Lisbon. Missionary activity began in earnest in 1540 when Francis Xavier arrived in Goa and subsequently made Kerala a hub of missionary activity. (The missionary activity was in no way a peaceful or benign one and hides several hideous facts, but I'll save this for a separate post.) Some time later, Christoph Clavius (1538-1612), a Jesuit mathematician and astronomer, included mathematics in the curriculum of Jesuit priests at Collegio Romano. Christoph Clavius was later to head the calendar reform committee of the church which resulted in the present Gregorian calendar. The first batch of Jesuit priests mathematically trained by Clavius reached Malabar in Kerala (including the city of Cochin, the epicenter of the Kerala mathematics) 1578 onwards. These included Matteo Ricci, Johann Schreck, and Antonio Rubino. These priests learnt the local language and were in close touch with local scholars and royal personages. Rubino and Ricci have also been recorded in correspondence as answering requests for astronomical information from the Kerala mathematical and astronomical sources. It is thus clear that the purpose of these mathematically trained priests is to acquire Indian knowledge navigation, astronomy, and the calendar. The Gregorian calendar reform took place soon after in 1582. In 1607, Christoph Clavius published tables of sines and cosines for different angles to a high degree of accuracy (seven to eight decimal places) without explaining how he carried out these calculations. (As mentioned earlier, these tables were important in navigation.)
Now, is it a mere coincidence that these tables had been already worked out by Madhava to the same degree of accuracy, and is it possible that Clavius's tables had nothing to do with Madhava's tables? Unlikely, considering the circumstantial evidence. Nilakantha quoted these tables of Madhava more than a century earlier in his Aryabhatia Bhashya which was an important text of the Kerala mathematics. (http://en.wikipedia.org/wiki/Madhava's_sine_table also shows below table. Also see  Madhava's Sine and Cosine Series by A. K. Bag.)
There is further circumstantial evidence that the knowledge of the Kerala mathematicians travelled to Europe. Tycho Brahe (1546-1601) was the most well known astronomer of the time who in 1597 became the Royal astronomer of the Holy Roman empire upon the invitation of emperor Rudolph II to Prague. He would thus be a natural recipient of any Hindu astronomical texts which might be obtained by the Jesuit priests in Kerala. Tycho Brahe is also well known for his model of planetary motion, in which Mercury, Venus, Mars, Jupiter and Saturn revolve around the sun, but the sun along with these planets revolve around the earth. Is it just a coincidence that exactly the same model was proposed by Nilakantha in his Tantra Sangraha?
Narayana Bhattathiri and the Narayaneeyam
As mentioned earlier, the Kerala scholars of Madhava's lineage were Nambudiri Brahmins and devotees of Lord Vishnu. The most important spiritual text for Lord Vishnu devotees is the Srimadbhagavad Puran, which consists of 18,000 verses and describes the life and deeds of Sri Krishna, an avatar of the Supreme. It seeks to increasing inner peace and develop dispassion towards the vicissitudes of life by reminding one of its impermance and putting one more and more in touch with the immortal, unchanging reality underlying the whole of creation. Sri Krishna also spoke the Bhagavad Gita to Arjuna on the eve of the Mahabharata war, in which three main paths are outline to realize this unchanging reality - Karma Yoga, Bhakti and Gyana Yoga. Srimadbhagavat Puran is especially valuable, indeed indispensable, for Bhakti Yogis, but is useful for Karma Yogis and Gyana Yogis as well.
One of the most important non-mathematical works of the Kerala scholars is the Narayaneeyam. This was written by Melpathur Narayana Bhattathiri (1559-1645) and is a condensed / abridged version of the Srimadbhagavat Puran. Melpathur was a linguist and was a student of Achyuta Pisharati.The legend associated with how he came to write the Narayaniyam is very beautiful and is well worth mentioning here. I am giving the legend below, which I have taken almost verbatim from the following website: www.narayaneeyam-firststep.org/introduction.html
Achyuta Pisharati, Melpathur Bhattathiri's Guru, suffered from acute rheumatism. Bhattathiri prayed to relieve his Guru of the disease and transfer it to him. His prayer was heard. His Guru regained health and instead the disease came to Narayana Bhattathiri. He was convinced that if he surrendered at the feet of Lord Krishna in the temple of Guruvayur he would be cured of disease. He sought the guidance of Thunchaththu Ramanujan Ezhuthachan in this matter. (Some information on Ezhuthachan: Thunchaththu Ramanujan Ezhuthachan was a renowned Malayalam devotional poet and linguist from that time. He was born in the Shudra caste in south Malabar and later on became a monk. He is considered the father of the Malayalam language and translated the Ramayana and the Mahabharata into Malayalam.) He instructed Bhattathiri to 'start with fish'. The suggestion would have seemed offensive to a Nambudiri Brahmin, who are strict vegetarians, but Bhattathiri understood the cryptic meaning. The fish is one of the avatars of Lord Vishnu described in the Bhagavat Puran (Matsya avatar - the fish avatar). Bhattathiri was quick to take the indication of glorifying the Lord with the various avatars starting with the Matsyaavatara. This prompted him to compose the abridged version of the Purana in he form of the Sriman Narayaneeyam Stotram.
In his acute rheumatism Bhattathiri somehow reached the temple of Guruvayoor and fully surrendered himself at the feet of the Lord. He prostrated in deep devotion and started to pray, singing His glory and worship Him everyday. It is said that he composed and rendered one dashakam (10 verses) each day to the Lord. Thus, in 100 days of sincere worship, the Narayaneeyam consisting of 100 dashakams was composed.
These verses written in pain and agony of the author invoked the sympathy and grace of the Lord. At the end of each dashakam, each day, he would pray for the mercy and kindness of the Lord to cure his disease. Soon the Lord's grace showered on him and on the 100th day the Lord blessed him with His vision. Bhattathiri was overwhelmed with ecstay and in the 100th Dashakam he cries out - 'Agre Pashyami' - which in Sanskrit means 'here I see Him in front of me' - and he gives a vivid description of the most enchanting form of the Lord he saw - from head to foot (Keshadi paadam). From that day his ailments vanished and he got totally cured.
 "On the Hindu quadrature of the circle, and the infinite series of the proportion of the circumference to the diameter exhibited in the four Sastras, the Tantra Sangraham, Yukti Bhasha, Carana Padhati, and Sadratnamala" by Charles M Whish, Transactions of the Royal Asiatic Society of Great Britain and Ireland, Vol. 3, No. 3 (1834).
 "Kerala school of astronomy and mathematics" by M. S. Sriram, Pg. 1160, "Encyclopedia of the History of Science, Technology, and Medicine in Non-Western Cultures", (ed. Helaine Selin, publ. Springer).
 "Ancient Indian leaps into Mathematics", B. S. Yadav and Manmohan (ed.) Birkhauser, Springer Science (2010).
C. T. Rajagopal and M. S. Rangachari, On an untapped source of medieval Keralese mathematics, Archive for history of exact sciences, Baltimore, Maryland, Vol 18 1978 pp. 89-102.
 Aryabhatiam with Bhashya of Nilakantha, edited by K. Sambasiva Sastri, Thiruvanathapuram Sanskrit Series, No. 101. Thiruvananthapuram (1930).
 "The discovery of the series formula for pi by Leibniz, Gregory, and Nilakantha", Ranjan Roy, Mathematics Magazine, Vol. 63, No. 5 (Dec. 1990), pp. 291-306, publ. Mathematical Association of America.
 Sadratnamala of Shankara Varman, critically edited by K. V. Sarma, Indian National Science Academy, New Delhi (2001).
 "The Golden Age of Indian Mathematics" by S. Parameswaran
 Glimpses of the history of mathematics in India, by K. Ramasubramanian, IIT Mumbai
 Development of calculus in India: contribution of kerala school (1350-1550 CE), K. Ramasubramanian, IIT Bombay
 Madhava's Sine and Cosine Series by A. K. Bag