Srinivasa Ramanujan. As they say, "The man who knew infinity".

Srinivasa Ramanujan is regarded as one of the greatest mathematicians ever in the world. He developed an immense interest in mathematics from a young age and met professor G. H. Hardy of Trinity College in the early 1910's. After receiving his degree from Cambridge University in 1916, Ramanujan started doing his own works and published several new theorems. Despite of having a short life-span and no formal training in pure mathematics, he brought a great revolution with his groundbreaking contributions to the field of mathematics.

1. Ramanujan's most important contribution to mathematics is his infinite series for pi

This series forms the basis of several algorithms used today to calculate π. He discovered many interesting formulas which help to calculate the digits of pi in several ways.

2. Ramanujan and Hardy did some research on partition function P(n) and introduced a non-convergent asymptotic series that allows the exact computation of the number of partitions of an integer. This idea led to the notion of Circle Method (an useful method to find asymptotic formulae).
3. Ramanujan introduced the idea of mock theta function which is a concept of mock modular form in mathematics. Although it remained a mystery for several years, now it is known as the holomorphic parts of mass form.
4. He made a huge contribution to the number theory. He discovered theta function and modular equations which are now widely used in number theory. His works on triangular number, sum of squares, partitions etc. have huge application to number theory in today's era. He introduced a lot of theorems on analytic number theory and his most notable publications in this field include 'Highly composite numbers' and "Proof that almost all numbers n are composed of about log(log n) prime factors".
5. Once when Ramanujan was hospitalized, Mr. G. H. Hardy came to visit him in a taxi number 1729. Hardy stated that it seemed to be a dull number, but Ramanujan replied that it's a very interesting number as it is the smallest number which can be written as sum of cubes of two different numbers in two different ways.

1729 = 1³ + 12³ = 9³ + 10³

Since then, 1729 is known as the 'Hardy - Ramanujan Number' and this idea later led to the notion of "Taxicab Numbers".

6. He introduced a number of new ideas that can be used to solve several hard mathematical problems. These ideas led to the development of game theory.
7. In his notebook, Ramanujan introduced over 2000 theorems and formulas of continued fractions of various functions.

Srinivasa Ramanujan was a genius in true sense. In his honour, 22nd December is celebrated as National Mathematics Day every year.