# Complex Variable Notes for GATE EE (Electrical Engineering) 2021 Exam

By BYJU'S Exam Prep

Updated on: September 25th, 2023

What is Analytic functions?

A complex function is said to be analytic on a region R if it is complex differentiable at every point in. The terms holomorphic function, differentiable function, and complex differentiable function are sometimes used interchangeably with “analytic function”.

Table of content

What is Analytic functions?

A complex function is said to be analytic on a region R if it is complex differentiable at every point in. The terms holomorphic function, differentiable function, and complex differentiable function are sometimes used interchangeably with "analytic function".

Cauchy-Riemann equations:

The Cauchy Riemann equations for a pair of given real-valued functions in two variables say u (x, y) and v (x, y) are the following two equations:

In a typical way, the values ‘u’ and ‘v’ are taken as the real and the imaginary parts of a complex-valued function of a single complex variable respectively,

z=x+iy, g(x+iy) = u(x,y)+iv(x,y)

if we are given that the functions u and v are differentiable at real values at a point in an open subset of the set of complex numbers that is C which can be taken as functions that are from R2 to R. This will imply that the partial derivatives of u and v do exist and thus we can also approximate smaller variations of ‘g’ in linear form. Then we say that g=u+iv  is differentiable at complex values at that particular point iff the Cauchy Riemann equations at that point are satisfied by the partial derivatives of u and v.

Cauchy Riemann Equations in Polar Coordinates:

### Cauchy Integral Theorem

If   is analytic in some simply connected region, then

……….(1)

for any closed contour  completely contained in . Writing  as

………..(2)

and  as

………(3)

then gives
Taylor Series

= ……….(4)

= ………..(5)

From Green's theorem,

=    ……….(6)

= …………(7)

But the Cauchy-Riemann equations require that

=   ………(8)

=  ……….(9)

So,    ……….(10)

Taylor Series for Holomorphic Functions. In Real Analysis, the Taylor series of a given function  f : R → R is given by:

We have examined some convergence issues and applications of Taylor series in MATH 2033/2043. We also learned that even if the function f is infinitely differentiable everywhere on R, its Taylor series may not converge to that function. In contrast, there is no such an issue in Complex Analysis: as long as the function f : C → C is holomorphic on an open ball B(z0), we can show the Taylor series of f.

## Laurent series

The Laurent series is a representation of a complex function f(z) as a series. Unlike the Taylor series which expresses f(z) as a series of terms with non-negative powers of z, a Laurent series includes terms with negative powers. A consequence of this is that a Laurent series may be used in cases where a Taylor expansion is not possible.

To calculate the Laurent series we use the standard and modified geometric series which are:

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