Leonhard Euler (1707-1783), Swiss mathematician. Euler was born at Basel, son of a church minister. He entered the university there at the age of 13 and also received tuition from Johann Bernoulli. In 1730 he became professor of physics at the University of St Petersburg and in 1741 he was invited to Berlin by Frederick the Great, where he spent 25 years before returning to Russia. His research revolutionised much of mathematics. He was not the mot rigorous in making proofs, but his insight led to major advances in the calculus, geometry and number theory. Euler was also introduced many notations including e as the base of natural logarithms, capital sigma as a summation symbol and i as the imaginary unit. He was totally blind in the last years of his life, but an ability to calculate in his head ensured that his productivity did not diminish.