power series
Dan Brown in his book, The Da Vinci Code, talks about the "divine proportion" as having a "fundamental role in nature". Brown's ideas are not completely without foundation, as the proportion crops up in the mathematics used to describe the formation of natural structures like snail's shells and plants, and even in Alan Turing's work on animal coats. But Dan Brown does not talk about mathematics, he talks about a number. What is so special about this number? 

Infinite series occupy a central and important place in mathematics. C. J. Sangwin shows us how eighteenthcentury mathematician Leonhard Euler solved one of the foremost infinite series problems of his day.
