Why does my graph come out wrong?

I want to plot a simple function, ((x.^2)-1).^(2/3). But my plot comes out looking different from the mathematicly correct one. Why is that? To specify, the graph should not be able to go under the x=0 line, yet it does.

[Jan]

Please post the code you use.

Answer 1

[Jan]

fplot(@(x) ((x.^2)-1).^(2/3))

[Ara Omar]

nevermind, i figured it out, i need an "abs" ahead of my function

Answer 2

[Voss]

x = -10:0.01:10;
y = (x.^2-1).^(2/3);
plot(x,y)

Warning: Imaginary parts of complex X and/or Y arguments ignored.

Note the warning about complex values. The values of y where abs(x) < 1 are complex because x^2-1 < 0 where abs(x) < 1. To skip plotting this reagin, you can keep track of when y is real and only plot those elements:

idx = imag(y) == 0;
plot(x(idx),y(idx))
ylim([-5 25])

Note the lack of warning this time. You could also replace the complex y with NaNs and plot that:

y(~idx) = NaN;
plot(x,y)
ylim([-5 25])

[Ara Omar]

When i added an "abs" infront of my function, it made the values of y when -1<x<1 all positive, which is what i was searching for.

[Voss]

If you want to plot abs(((x.^2)-1).^(2/3)), then that will work.