A central connection between PDE and the mainstream of mathematical development in the 19th century arose from the role of PDE in the theory of analytic functions of a complex variable. Cauchy had observed in 1827 that two smooth real functions u, v of two real variables x, y are the real and imaginary parts of a single analytic complex function of the complex variable z=x+iy if they satisfy the Cauchy-Riemann system of first order equations: