Plotting complex roots of polynomial

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Thomas Sun 2020-5-19

评论： David Goodmanson 2020-5-19

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

David Goodmanson 2020-5-19

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 2020-5-19

Hi Dave

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

David Goodmanson 2020-5-19

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 ....?

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