Euler worked in almost all areas of mathematics: geometry, calculus, trigonometry, algebra, and number theory, not to mention continuum physics, lunar theory and other areas of physics. His importance in the history of mathematics cannot be overstated: if printed, his works, many of which are of fundamental interest, would occupy between 60 and 80 quarto volumes and Euler's name is associated with an impressive number of topics. The 20th century Hungarian mathematician Paul Erdős is perhaps the only other mathematician who could be considered to be as prolific.

Euler introduced and popularized several notational conventions through his numerous and widely circulated textbooks. Most notably, he introduced the concept of a function and was the first to write f(x) to denote the function f applied to the argument x. He also introduced the modern notation for the trigonometric functions, the letter e for the base of the natural logarithm (now also known as Euler's number), the Greek letter Σ for summations and the letter i to denote the imaginary unit. The use of the Greek letter π to denote the ratio of a circle's circumference to its diameter was also popularized by Euler, although it did not originate with him.

Euler's great interest in number theory can be traced to the influence of his friend in the St. Petersburg Academy, Christian Goldbach. A lot of his early work on number theory was based on the works of Pierre de Fermat. Euler developed some of Fermat's ideas while disproving some of his more outlandish conjectures.

One focus of Euler's work was to link the nature of prime distribution with ideas in analysis. He proved that the sum of the reciprocals of the primes diverges. In doing so, he discovered the connection between Riemann zeta function and prime numbers, known as the Euler product formula for the Riemann zeta function.

Euler proved Newton's identities, Fermat's little theorem, Fermat's theorem on sums of two squares, and made distinct contributions to Lagrange's four-square theorem. He also invented the totient function φ(n) which assigns to a positive integer n the number of positive integers less than n and coprime to n. Using properties of this function he was able to generalize Fermat's little theorem to what would become known as Euler's theorem. He further contributed significantly to the understanding of perfect numbers, which had fascinated mathematicians since Euclid. Euler made progress toward the prime number theorem and conjectured the law of quadratic reciprocity. The two concepts are regarded as the fundamental theorems of number theory, and his ideas paved the way for the work of Carl Friedrich Gauss.

Source: www.wikipedia.org