- #1
SqueeSpleen
- 141
- 5
Determine the quantitiy of zeroes of the function:
[itex]f(z)=z^{4}-8z+10[/itex]
a) Inside the circle [itex]| z | < 1[/itex]
b) Inside the ring [itex]1 \leq | z | < 2[/itex]a)
[itex]f(z)=(z^{4}-8z)+10=g(z)+h(z)[/itex]
As [itex]|h(z)| \geq |g(z)| \forall z : | z | = 1[/itex]
Then by Rouche's Theorem the number of zeros of the function inside the circle is the same than [itex]h(z)[/itex] (0 zeroes).
b) My idea was to calculate the number of zeroes in [itex]| z | < 2[/itex] and substract the number of zeroes in [itex]| z | < 1[/itex].
But I can't find a pair of functions to use Roche's Theorem.
Any hint?
I ploted here:
http://www.wolframalpha.com/input/?i=z^4-8z%2B10
And I'm starting to suspect that the statement is wrong, the roots are very close to the circle and I guess that this is what make the functions so hard to find.
But I may be wrong and perhaps it's a easy way to decompose [itex]f(z)[/itex]
[itex]f(z)=z^{4}-8z+10[/itex]
a) Inside the circle [itex]| z | < 1[/itex]
b) Inside the ring [itex]1 \leq | z | < 2[/itex]a)
[itex]f(z)=(z^{4}-8z)+10=g(z)+h(z)[/itex]
As [itex]|h(z)| \geq |g(z)| \forall z : | z | = 1[/itex]
Then by Rouche's Theorem the number of zeros of the function inside the circle is the same than [itex]h(z)[/itex] (0 zeroes).
b) My idea was to calculate the number of zeroes in [itex]| z | < 2[/itex] and substract the number of zeroes in [itex]| z | < 1[/itex].
But I can't find a pair of functions to use Roche's Theorem.
Any hint?
I ploted here:
http://www.wolframalpha.com/input/?i=z^4-8z%2B10
And I'm starting to suspect that the statement is wrong, the roots are very close to the circle and I guess that this is what make the functions so hard to find.
But I may be wrong and perhaps it's a easy way to decompose [itex]f(z)[/itex]