# Complex number inequality

## Homework Statement

Let $$z,w$$ be complex numbers.

## Homework Equations

Prove there is a real number $$\alpha < 1$$ such that
$$\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right| \leq \alpha \left|z - w\right|$$

The goal is to show that $$\displaystyle q(z) = \frac{z^7 + z^3 - i}{9}$$ is a contraction mapping in a real analysis contraction mapping problem. I am stuck here maybe because of algebraic manipulation.

## The Attempt at a Solution

Is this the same as proving the following inequality?
\begin{align*} \frac{\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right|}{\left|z - w\right|} < 1 \end{align*}

If so then
\begin{align*} & \frac{\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right|}{\left|z - w\right|} = \left|\frac{\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}}{z - w}\right| = \left|\frac{\frac{(z^7 + z^3) - (w^7 + w^3)}{9}}{z-w}\right| = \frac{1}{9} \left|\frac{(z^7 + z^3) - (w^7 + w^3)}{z-w}\right| \\ = & \frac{1}{9} \left|\frac{z^7 - w^7 + z^3 - w^3}{z-w}\right| = \frac{1}{9} \left| \frac{(z-w)(z^6 + z^5w + \cdots + zw^5 + w^6) + (z-w)(z^2 + zw + w^2)}{z-w} \right| \\ =& \frac{1}{9} |(z^6 + z^5w + z^4w^2 + z^3w^3 + z^2w^4 + zw^5 + w^6) + (z^2 + zw + w^2)| \end{align*}

How can I proceed from here?

Mark44
Mentor

## Homework Statement

Let $$z,w$$ be complex numbers.

## Homework Equations

Prove there is a real number $$\alpha < 1$$ such that
$$\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right| \leq \alpha \left|z - w\right|$$

The goal is to show that $$\displaystyle q(z) = \frac{z^7 + z^3 - i}{9}$$ is a contraction mapping in a real analysis contraction mapping problem. I am stuck here maybe because of algebraic manipulation.

## The Attempt at a Solution

Is this the same as proving the following inequality?
\begin{align*} \frac{\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right|}{\left|z - w\right|} < 1 \end{align*}

If so then
\begin{align*} & \frac{\left|\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}\right|}{\left|z - w\right|} = \left|\frac{\frac{z^7 + z^3 - i}{9} - \frac{w^7 + w^3 - i}{9}}{z - w}\right| = \left|\frac{\frac{(z^7 + z^3) - (w^7 + w^3)}{9}}{z-w}\right| = \frac{1}{9} \left|\frac{(z^7 + z^3) - (w^7 + w^3)}{z-w}\right| \\ = & \frac{1}{9} \left|\frac{z^7 - w^7 + z^3 - w^3}{z-w}\right| = \frac{1}{9} \left| \frac{(z-w)(z^6 + z^5w + \cdots + zw^5 + w^6) + (z-w)(z^2 + zw + w^2)}{z-w} \right| \\ =& \frac{1}{9} |(z^6 + z^5w + z^4w^2 + z^3w^3 + z^2w^4 + zw^5 + w^6) + (z^2 + zw + w^2)| \end{align*}

How can I proceed from here?

Are there some conditions on z and w that you don't show and aren't using? It doesn't seem to me that q(z) = (z7 + z3 - i)/9 is a contraction mapping, in general. For example, q(2+0i) = (128 + 8 - i)/9 has a magnitude considerably larger than 2. If q were a contraction mapping, I would expect |q(z)| <= |z|.