That is, the real and imaginary parts of …give solutions to a simultaneous system of first-order partial differential equation : the Cauchy-Riemann equations of course. This is much stronger than the mere existence of the partial derivatives.
Notice, therefore, that the property of being analytic for a complex function …….. is much stronger than that of being continuously differentiable in the superficially analogous case of a real function ….. Moreover, this is in spite of the fact that the definition of… via the difference quotient looks the same in each case. The existence of the two-dimensional complex limit of the difference quotient as…approaches ..is a powerful hypothesis.