# Taylor Series for Functions of a Complex Variable

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Lesson Transcript
Instructor: Gerald Lemay

Gerald has taught engineering, math and science and has a doctorate in electrical engineering.

This lesson explores how analytic functions can be expressed as Taylor series. We use the concepts of complex differentiable functions and Cauchy-Riemann equations.

## Taylor Series: Complex Variables

You probably know the formula for finding the Taylor series of a function like f(x) = 1/(1 - x). Using the Taylor series formula we get:

For clarity, x is a real variable, meaning it can have values on the number line, and the summation formula on the right-hand side is a compact way of writing 1 + x + x2 + ... where n takes values from 0 to âˆž. The more terms we add, the better our approximation to the actual function. But there is a catch! We need more information.

For example, with 1/(1 - x), the summation formula only works for |x| < 1. The last part, |x| < 1, tells us where on the x-axis we can pick values (like x = 0.9) and still be able to use the summation expression. This is called the region of convergence. If we pick any value for x outside of the region of convergence (like x = 1.1), the summation will not represent the function, f(x), no matter how many terms we add together.

What if instead of a real variable, x, we have a complex variable, z ?

In this lesson we explore this question using the ideas of complex differentiability, Cauchy-Riemann conditions, analytic functions, and a region of convergence defined by a circle instead of a number line.

## Complex Differentiability

If you can do basic manipulations of complex numbers and calculate partial derivatives, you can test for complex differentiability. Here are the steps:

First, write the function in the form:

identifying u(x,y) and v(x,y).

Then, calculate 4 partial derivatives:

Let's do an example with f(z) = 1/(1 - z).

#### Step 1: Write f(z) in the form f(x,y) = u(x,y) + i v(x,y)

We have f(z) = 1/(1 - z). But z is complex, meaning z = x + iy.

Substitute x + iy for z:

Which can be written as:

The denominator has a real part, 1 - x, and an imaginary part, -y.

Next, multiply the numerator and denominator by 1 - x + iy (this is sometimes called ''rationalizing the denominator''):

Separate the real parts and the imaginary parts:

Comparing this result to:

tells us:

and

We've done the first step!

#### Step 2: Calculate partial derivatives.

The partial derivative of u(x,y) with respect to x means x is the variable and everything else is a constant. When we differentiate with respect to y, x is held constant. Using the quotient rule for differentiation:

This simplifies to:

Now, differentiate v(x,y) with respect to y:

which simplifies to:

Two more to go. Partial derivative of v(x,y) with respect to x:

which simplifies to:

Last one: differentiate u(x,y) with respect to y:

Simplifying:

This completes step 2.

Now we compare these results. If:

and:

then, the function f(z) = 1/(1 - z) is an analytic function.

These two conditions are true, and indeed, our function is an analytic function. By the way, these two test equations are called the Cauchy-Riemann equations, which are used when exploring analytic functions.

## Taylor Series: Analytic Functions

The formal statement goes like this:

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