Complex Function Theory
The complex numbers, described by Leibniz as amphibia between existence and non-existence, are now an important tool for both pure and applied mathematics. They have a fruitful geometric interpretation, provide an algebraic closure to the reals (in the sense that all polynomials with coefficient in C have roots in C), and allow, with a more coherent theory than for real variables, the development of the calculus. The important exponential function, in particular, extends elegantly to the complex domain.