Euler, Leonhard (lā`ônhärt oi`lər), 1707–83, Swiss mathematician. Born and educated at Basel, where he knew the Bernoullis, he went to St. Petersburg (1727) at the invitation of Catherine I, becoming professor of mathematics there on the departure of Daniel Bernoulli (1733). He was invited to Berlin (1741) by Frederick the Great and remained there until 1766, when he returned to St. Petersburg. Euler was the most prolific mathematician who ever lived; his collected works run to more than seventy volumes. He contributed to numerous areas of both pure and applied mathematics, including the calculus of variations, analysis, number theory, algebra, geometry, trigonometry, analytical mechanics, hydrodynamics, and the lunar theory (calculation of the motion of the moon). Euler was one of the first to develop the methods of the calculus calculus, branch of mathematics that studies continuously changing quantities. The calculus is characterized by the use of infinite processes, involving passage to a limit —the notion of tending toward, or approaching, an ultimate value.
..... Click the link for more information. on a wide scale. Though half-blind for much of his life and totally blind for the last seventeen years, he retained to the end a near-legendary skill at calculation. Among his results are the differential equation named for him, the formula relating the number of faces, edges, and vertices of a polyhedron (F + V = E + 2), and the famous equation e^iπ + 1 = 0 connecting five fundamental numbers in mathematics.

Euler, Leonhard

(born April 15, 1707, Basel, Switz.—died Sept. 18, 1783, St. Petersburg, Russia) Swiss mathematician. In 1733 he succeeded Daniel Bernoulli (see Bernoulli family) at the St. Petersburg Academy of Sciences. There he developed the theory of trigonometric and logarithmic functions and advanced mathematics generally. Under the patronage of Frederick the Great, he worked at the Berlin Academy for many years (1744–66), where he developed the concept of function in mathematical analysis and discovered the imaginary logarithms of negative numbers. Throughout his life he was interested in number theory. In addition to inspiring the use of arithmetic terms in writing mathematics and physics, Euler introduced many symbols that became standard, including ∑ for summation; ∫n for the sum of divisors of n; e for the base of the natural logarithm; a, b, and c for the sides of a triangle with A, B, and C for the opposite angles; f(x) for a function; π for the ratio of the circumference to the diameter of a circle; and i for √−1. He is considered one of the greatest mathematicians of all time.