# Solving $(-1-z)^n = z$, for $z$ in the unit circle

I want to solve the following equation:

$$(-1-z)^n = z,\quad |z|<1,\quad n>1\in \mathbb{N}$$

where $z$ is a complex number.

However Solve or Reduce didn't get me anywhere. Is there a way to do this?

• "Solve or Reduce didn't get me anywhere" what exactly did you try? – gpap Jun 20 '13 at 9:11

## 3 Answers

I would still like to see your effort before answering because Reduce works fine:

f[n_] := Last /@ Reduce[(-z - 1)^n == z && Abs@z < 1, {z}] /. Or -> List


(essentially reproducing what Simon Woods did).

{Re[#], Im[#]} & /@ N[f];
Show[Graphics@Circle[], ListPlot[%]] • I think Hannah had tried Reduce with symbolic n – Simon Woods Jun 20 '13 at 10:14
• @gpap nice graphic! those dots should all be centered in a circle at -1 on the real axis, though - do you have any idea why the two dots in the middle seem to be slightly off? – Vincent Tjeng Jun 20 '13 at 10:33
• that's very nice! i am at the office right now and what i tried myself is hon my home computer; but i can show you as soon as i'm home – Hannah Jun 20 '13 at 11:22
• @VincentTjeng I think we can very roughly think like this. Let $w=z+1$, then the original eq can be reduced to $(-1)^n w^{n-1}=1-1/w$, where $(-1)^n w^{n-1}=1$ is exactly on the circle $\left\| z+1\right\| =1$. So the more $w$ near $1$, the more perturbation it introduces to the radius of the circle. – Silvia Jun 20 '13 at 12:21

Since your equation is a polynomial, you could just create a list of Root objects for a given value of n and select those within the unit circle:

solution[n_] :=
Select[Table[Root[(-1 - z)^n - z, i], {i, n}], Abs[#] < 1 &];

solution // N
(*  {-0.203456, -0.294702 - 0.637624 I, -0.294702 + 0.637624 I}  *)

With[{z = x + I y, n = 7},
DensityPlot[Log@Abs[(-1 - z)^n - z], {x, -1, 1}, {y, -1, 1},
Epilog -> {Circle[], Circle[{Re[#], Im[#]}, 0.1] & /@ solution[n]}]] • Thank's a lot! would it be possible to creat a loop around this expression so that it is calculated for let's say n from 1 to 100 or so? – Hannah Jun 20 '13 at 9:38

A partial answer first - I don't think it's possible to get general solutions as a function of $n$, especially as the number of solutions will change depending on the value of $n$. My attempt in Mathematica as shown below to solve this was met by the error code Solve::nsmet: This system cannot be solved with the methods available to Solve. >>

Solve[(-z - 1)^n == z && Abs[z] < 1 && n \[Element] Integers, {z}]


However, if you're only interested to know the approximate value of the solutions for a given value of $n$, you can try the following line of code (the following example evaluated for when $n$ is 6:

n=6;
NSolve[(-z - 1)^n == z && Abs[z] < 1, {z}]


Tell me if this meets your needs!

• $-(z+1)^n \neq (-1-z)^n$ for even $n$. – shrx Jun 20 '13 at 9:07
• @shrx - thanks! missed out on a pair of brackets, edited as you suggest. – Vincent Tjeng Jun 20 '13 at 9:18