Plotting complex roots of polynomial

Hi I need to plot of the roots of a polynomial in the complex plane. This is the code I have so far but it doesn't seem to be working.

z = -1:0.01:1;

p = z.^9 + z.^8 - z.^7 + z.^6 - z.^5 + z + 1;

plot(z,p)

hold on

r = roots(p);

scatter(real(r),imag(r))

hold off

Answer 1

[David Goodmanson]

Hi Thomas,

what the roots function is looking for is the coefficients of the powers of z in decreasing order. In this case that would be

r = roots([1 1 -1 1 -1 0 0 0 1 1])

This shows one real root at approximately -1.9, and eight complex roots. If you use

z = -2:0.01:1; 

you can see the curve go through the real root.

[Thomas Sun]

Hi David

Thanks for the response

I tried this but it didn't seem to work - the complex roots don't seem to come up

[David Goodmanson]

here is the code I used

z = -2:0.01:1;
p = z.^9 + z.^8 - z.^7 + z.^6 - z.^5 + z + 1;
r = roots([1 1 -1 1 -1 0 0 0 1 1])
plot(z,p);
hold on
plot(r,'o') 
% scatter(real(r),imag(r))   % this one works too
hold off

[Thomas Sun]

Hi Dave

I tried your code but with it a graph for p doesnt even show up

[David Goodmanson]

what do you get for the sizes of z and p?

maybe if you tried 'close all' and put 'figure(1)' just before the plot command ....?