Does $z^4+2z^2+z=0$ have complex roots?
Does $z^4+2z^2+z=0$ have complex roots? How to find them? Besides $z=0$, I got the equation $re^{3itheta}+2re^{itheta}=e^{i(pi+2kpi)}$, $kin mathbb Z$. How to find the complex roots?
calculus complex-numbers
add a comment |
Does $z^4+2z^2+z=0$ have complex roots? How to find them? Besides $z=0$, I got the equation $re^{3itheta}+2re^{itheta}=e^{i(pi+2kpi)}$, $kin mathbb Z$. How to find the complex roots?
calculus complex-numbers
3
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
4
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26
add a comment |
Does $z^4+2z^2+z=0$ have complex roots? How to find them? Besides $z=0$, I got the equation $re^{3itheta}+2re^{itheta}=e^{i(pi+2kpi)}$, $kin mathbb Z$. How to find the complex roots?
calculus complex-numbers
Does $z^4+2z^2+z=0$ have complex roots? How to find them? Besides $z=0$, I got the equation $re^{3itheta}+2re^{itheta}=e^{i(pi+2kpi)}$, $kin mathbb Z$. How to find the complex roots?
calculus complex-numbers
calculus complex-numbers
asked Nov 29 at 3:00
user398843
598215
598215
3
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
4
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26
add a comment |
3
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
4
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26
3
3
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
4
4
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26
add a comment |
2 Answers
2
active
oldest
votes
Enough to check the discriminant for $ax^3+bx^2+cx+d=0$:
$$
Delta_3=-27 a^2 d^2+18 a b c d-4 a c^3-4 b^3 d+b^2 c^2
$$
$$
Delta_3=begin{cases} >0 & text{3 distinct real roots}\
<0 & text{1 real, 2 conjugate complex roots}\
=0 & text{3 real roots with duplicates}\
end{cases}
$$
In your case, it's $a=1$, $b=0$, $c=2$ and $d=1$, so $Delta_3=-59$, hence the equation has one real and two complex conjugate roots, with all 3 distinct. As for finding the roots themselves, the Cardano formula helps check this out. The final result is:
$$
x_1=frac{sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{3^{2/3}}-2 sqrt[3]{frac{2}{3
left(sqrt{177}-9right)}}
$$
$$
x_2=left(1+i sqrt{3}right)
sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1-i sqrt{3}right)
sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2 3^{2/3}}
$$
$$
x_3=left(1-i
sqrt{3}right) sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1+i
sqrt{3}right) sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2
3^{2/3}}
$$
add a comment |
According to Wolfy,
the roots are
$0, ≈-0.453397651516404,
≈0.22669882575820 pm 1.46771150871022 i
$
so the answer is yes.
Another way to see this is that,
if
$f(z) = z^4+2z^2+z$,
then
$f'(z)
= 4z^3+4z+1
$
and
$f''(z)
= 12z^2+4
gt 0
$
so $f(z)$
can have at most two real roots.
Since
$f(0) = 0$
and $f'(0)=1 ne 0$,
$f(z)$ has exactly
two real roots
so has
two (conjugate) complex roots
since all its coefficients
are real.
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
Enough to check the discriminant for $ax^3+bx^2+cx+d=0$:
$$
Delta_3=-27 a^2 d^2+18 a b c d-4 a c^3-4 b^3 d+b^2 c^2
$$
$$
Delta_3=begin{cases} >0 & text{3 distinct real roots}\
<0 & text{1 real, 2 conjugate complex roots}\
=0 & text{3 real roots with duplicates}\
end{cases}
$$
In your case, it's $a=1$, $b=0$, $c=2$ and $d=1$, so $Delta_3=-59$, hence the equation has one real and two complex conjugate roots, with all 3 distinct. As for finding the roots themselves, the Cardano formula helps check this out. The final result is:
$$
x_1=frac{sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{3^{2/3}}-2 sqrt[3]{frac{2}{3
left(sqrt{177}-9right)}}
$$
$$
x_2=left(1+i sqrt{3}right)
sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1-i sqrt{3}right)
sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2 3^{2/3}}
$$
$$
x_3=left(1-i
sqrt{3}right) sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1+i
sqrt{3}right) sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2
3^{2/3}}
$$
add a comment |
Enough to check the discriminant for $ax^3+bx^2+cx+d=0$:
$$
Delta_3=-27 a^2 d^2+18 a b c d-4 a c^3-4 b^3 d+b^2 c^2
$$
$$
Delta_3=begin{cases} >0 & text{3 distinct real roots}\
<0 & text{1 real, 2 conjugate complex roots}\
=0 & text{3 real roots with duplicates}\
end{cases}
$$
In your case, it's $a=1$, $b=0$, $c=2$ and $d=1$, so $Delta_3=-59$, hence the equation has one real and two complex conjugate roots, with all 3 distinct. As for finding the roots themselves, the Cardano formula helps check this out. The final result is:
$$
x_1=frac{sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{3^{2/3}}-2 sqrt[3]{frac{2}{3
left(sqrt{177}-9right)}}
$$
$$
x_2=left(1+i sqrt{3}right)
sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1-i sqrt{3}right)
sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2 3^{2/3}}
$$
$$
x_3=left(1-i
sqrt{3}right) sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1+i
sqrt{3}right) sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2
3^{2/3}}
$$
add a comment |
Enough to check the discriminant for $ax^3+bx^2+cx+d=0$:
$$
Delta_3=-27 a^2 d^2+18 a b c d-4 a c^3-4 b^3 d+b^2 c^2
$$
$$
Delta_3=begin{cases} >0 & text{3 distinct real roots}\
<0 & text{1 real, 2 conjugate complex roots}\
=0 & text{3 real roots with duplicates}\
end{cases}
$$
In your case, it's $a=1$, $b=0$, $c=2$ and $d=1$, so $Delta_3=-59$, hence the equation has one real and two complex conjugate roots, with all 3 distinct. As for finding the roots themselves, the Cardano formula helps check this out. The final result is:
$$
x_1=frac{sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{3^{2/3}}-2 sqrt[3]{frac{2}{3
left(sqrt{177}-9right)}}
$$
$$
x_2=left(1+i sqrt{3}right)
sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1-i sqrt{3}right)
sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2 3^{2/3}}
$$
$$
x_3=left(1-i
sqrt{3}right) sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1+i
sqrt{3}right) sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2
3^{2/3}}
$$
Enough to check the discriminant for $ax^3+bx^2+cx+d=0$:
$$
Delta_3=-27 a^2 d^2+18 a b c d-4 a c^3-4 b^3 d+b^2 c^2
$$
$$
Delta_3=begin{cases} >0 & text{3 distinct real roots}\
<0 & text{1 real, 2 conjugate complex roots}\
=0 & text{3 real roots with duplicates}\
end{cases}
$$
In your case, it's $a=1$, $b=0$, $c=2$ and $d=1$, so $Delta_3=-59$, hence the equation has one real and two complex conjugate roots, with all 3 distinct. As for finding the roots themselves, the Cardano formula helps check this out. The final result is:
$$
x_1=frac{sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{3^{2/3}}-2 sqrt[3]{frac{2}{3
left(sqrt{177}-9right)}}
$$
$$
x_2=left(1+i sqrt{3}right)
sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1-i sqrt{3}right)
sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2 3^{2/3}}
$$
$$
x_3=left(1-i
sqrt{3}right) sqrt[3]{frac{2}{3 left(sqrt{177}-9right)}}-frac{left(1+i
sqrt{3}right) sqrt[3]{frac{1}{2} left(sqrt{177}-9right)}}{2
3^{2/3}}
$$
answered Nov 29 at 3:11
Ákos Somogyi
1,547417
1,547417
add a comment |
add a comment |
According to Wolfy,
the roots are
$0, ≈-0.453397651516404,
≈0.22669882575820 pm 1.46771150871022 i
$
so the answer is yes.
Another way to see this is that,
if
$f(z) = z^4+2z^2+z$,
then
$f'(z)
= 4z^3+4z+1
$
and
$f''(z)
= 12z^2+4
gt 0
$
so $f(z)$
can have at most two real roots.
Since
$f(0) = 0$
and $f'(0)=1 ne 0$,
$f(z)$ has exactly
two real roots
so has
two (conjugate) complex roots
since all its coefficients
are real.
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
add a comment |
According to Wolfy,
the roots are
$0, ≈-0.453397651516404,
≈0.22669882575820 pm 1.46771150871022 i
$
so the answer is yes.
Another way to see this is that,
if
$f(z) = z^4+2z^2+z$,
then
$f'(z)
= 4z^3+4z+1
$
and
$f''(z)
= 12z^2+4
gt 0
$
so $f(z)$
can have at most two real roots.
Since
$f(0) = 0$
and $f'(0)=1 ne 0$,
$f(z)$ has exactly
two real roots
so has
two (conjugate) complex roots
since all its coefficients
are real.
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
add a comment |
According to Wolfy,
the roots are
$0, ≈-0.453397651516404,
≈0.22669882575820 pm 1.46771150871022 i
$
so the answer is yes.
Another way to see this is that,
if
$f(z) = z^4+2z^2+z$,
then
$f'(z)
= 4z^3+4z+1
$
and
$f''(z)
= 12z^2+4
gt 0
$
so $f(z)$
can have at most two real roots.
Since
$f(0) = 0$
and $f'(0)=1 ne 0$,
$f(z)$ has exactly
two real roots
so has
two (conjugate) complex roots
since all its coefficients
are real.
According to Wolfy,
the roots are
$0, ≈-0.453397651516404,
≈0.22669882575820 pm 1.46771150871022 i
$
so the answer is yes.
Another way to see this is that,
if
$f(z) = z^4+2z^2+z$,
then
$f'(z)
= 4z^3+4z+1
$
and
$f''(z)
= 12z^2+4
gt 0
$
so $f(z)$
can have at most two real roots.
Since
$f(0) = 0$
and $f'(0)=1 ne 0$,
$f(z)$ has exactly
two real roots
so has
two (conjugate) complex roots
since all its coefficients
are real.
answered Nov 29 at 6:05
marty cohen
72.3k549127
72.3k549127
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
add a comment |
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I think OP is asking for complex, not real roots.
– YiFan
Nov 29 at 13:14
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
I showed that it has two real and two complex roots. This exactly answers the question. Did you not read my last sentence?
– marty cohen
Nov 29 at 16:20
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
The question asks, "Does $z^2+2z^2+z=0$ have complex roots? [...] How to find the complex roots?" Your answer addresses only the first part and the rest is done with Wolfy.
– YiFan
Nov 29 at 20:34
add a comment |
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3
By the fundamental theorem of algebra, every polynomial of degree $n$ has exactly $n$ roots in $mathbb{C}$. As for finding yours, though, I'm not sure.
– Eevee Trainer
Nov 29 at 3:01
4
The polynomial is $zP$ where $P = z^3+2z+1$. Since $frac{dP}{dz} = 3z^2 + 2$ is positive for all $z$, it follows that $P$ is an increasing function and can have only one real root.
– Théophile
Nov 29 at 3:17
Theophile's answer is by far the best here. Note we can get even more via a quick application of Rouche's Theorem, which tells us each root must be the disk $|z| le 2$ (since on the circle of radius two, $|2z^2 + z| le 2|z|^2 + |z| = 10 and $|z^4| = 16,$ we conclude all four roots lie in the disk.
– Brevan Ellefsen
Nov 29 at 9:26