Values of $a$ for which $p(x)$ has a complex roots?
Let $p(x) = x^4 + ax^3 + bx^2 + ax + 1$. Suppose $x = 1$ is a root of $p(x)$, then find the range of values of $a$ such that $p(x)$ has complex (non-real) roots?
My approach: Using the fact that $1$ is a root, I am able to deduce $b = -2(a+1)$. Then I wrote $$p(x) = (x-1)(x-z)(x-bar{z})(x-k)$$ where $z = c + id, dneq 0$ and $k$ is some other real roots. Now I plan to expand and compare the coefficients but have been unable to do so in a fruitful manner (there just too many variables to get to a proper estimate of range of $a$ )
polynomials
add a comment |
Let $p(x) = x^4 + ax^3 + bx^2 + ax + 1$. Suppose $x = 1$ is a root of $p(x)$, then find the range of values of $a$ such that $p(x)$ has complex (non-real) roots?
My approach: Using the fact that $1$ is a root, I am able to deduce $b = -2(a+1)$. Then I wrote $$p(x) = (x-1)(x-z)(x-bar{z})(x-k)$$ where $z = c + id, dneq 0$ and $k$ is some other real roots. Now I plan to expand and compare the coefficients but have been unable to do so in a fruitful manner (there just too many variables to get to a proper estimate of range of $a$ )
polynomials
add a comment |
Let $p(x) = x^4 + ax^3 + bx^2 + ax + 1$. Suppose $x = 1$ is a root of $p(x)$, then find the range of values of $a$ such that $p(x)$ has complex (non-real) roots?
My approach: Using the fact that $1$ is a root, I am able to deduce $b = -2(a+1)$. Then I wrote $$p(x) = (x-1)(x-z)(x-bar{z})(x-k)$$ where $z = c + id, dneq 0$ and $k$ is some other real roots. Now I plan to expand and compare the coefficients but have been unable to do so in a fruitful manner (there just too many variables to get to a proper estimate of range of $a$ )
polynomials
Let $p(x) = x^4 + ax^3 + bx^2 + ax + 1$. Suppose $x = 1$ is a root of $p(x)$, then find the range of values of $a$ such that $p(x)$ has complex (non-real) roots?
My approach: Using the fact that $1$ is a root, I am able to deduce $b = -2(a+1)$. Then I wrote $$p(x) = (x-1)(x-z)(x-bar{z})(x-k)$$ where $z = c + id, dneq 0$ and $k$ is some other real roots. Now I plan to expand and compare the coefficients but have been unable to do so in a fruitful manner (there just too many variables to get to a proper estimate of range of $a$ )
polynomials
polynomials
asked Nov 28 '18 at 1:37
henceproved
1358
1358
add a comment |
add a comment |
4 Answers
4
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HINT:
$$p(x)=(x-1)(x^3+(a+1)x^2-(a+1)x-1)$$
Rest follows very nicely.
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
add a comment |
When a polynomial is "palindromic", reading the same way forwards and backwards, it must also have palindromic factors. Thus
$x^4+ax^3+bx^2+ax+1=(x^2+cx+1)(x^2+dx+1)$
If $x=1$ is to be a root then it must give $c=-2$ or $d=-2$ in one of the factors, as no other choice has the root $x=1$. Taking $c$ as $-2$ we then get the complex roots if $-2<d<2$, corresponding to a negative discriminate for the $x^2+dx+1$ factor.
Expanding the product $(x^2+cx+1)(x^2+dx+1)$ and equating this to $x^4+ax^3+bx^2+ax+1$ then implies $a=c+d=d-2$ so the solution to the problem is $-4<a<0$.
add a comment |
Here's a different approach. (I can't say for sure whether you can or cannot do it by comparing coefficients though!)
For a degree 2 polynomial, we know that it has complex roots (assuming the coefficients are real) iff the discriminant is negative. There's a similar generalisation for high degree polynomial.
In particular, a degree 4 polynomial
$$ax^4+bx^3+cx^2+dx+e$$
has discriminant
$$256a^3e^3-192a^2bde^2-128a^2c^2e^2+144a^2d^2e...$$
(see https://en.wikipedia.org/wiki/Discriminant#Degree_4 for the full formula). And we know that since $x=1$ is a root there can only be a pair of complex roots so we require the discriminant to be negative.
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
add a comment |
Considering that $$p(x) = x^4 + ax^3 + bx^2 + ax + 1$$ If $x=1$ is a root, then
$$p(1)=2 a+b+2=0 implies b=-2-2a$$ and
$$p(x)=(x-1),(x^3+(a+1) x^2-(a+1) x-1)$$ Clearly, $x=1$ is also a root for the second term making
$$p(x)=(x-1)^2,(x^2+(a+2) x+1)$$ So, there are complex roots if the discriminant $Delta$ of the remaining quadratic equation is negative.
add a comment |
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4 Answers
4
active
oldest
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4 Answers
4
active
oldest
votes
active
oldest
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active
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votes
HINT:
$$p(x)=(x-1)(x^3+(a+1)x^2-(a+1)x-1)$$
Rest follows very nicely.
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
add a comment |
HINT:
$$p(x)=(x-1)(x^3+(a+1)x^2-(a+1)x-1)$$
Rest follows very nicely.
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
add a comment |
HINT:
$$p(x)=(x-1)(x^3+(a+1)x^2-(a+1)x-1)$$
Rest follows very nicely.
HINT:
$$p(x)=(x-1)(x^3+(a+1)x^2-(a+1)x-1)$$
Rest follows very nicely.
answered Nov 28 '18 at 1:58
Rhys Hughes
4,8041327
4,8041327
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
add a comment |
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
If you don't see how, theres a solution to $$x^3+(a+1)x^2-(a+1)x-1=0$$ that you should be able to spot immediately.
– Rhys Hughes
Nov 28 '18 at 2:03
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
$1$ is a mutiple root and after that I have to show that determinant is $leq 0$ for the remaining quadritic term.
– henceproved
Nov 28 '18 at 2:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
Absolutely, except the inequality is strict $(<0$ rather than $le 0)$
– Rhys Hughes
Nov 28 '18 at 5:09
add a comment |
When a polynomial is "palindromic", reading the same way forwards and backwards, it must also have palindromic factors. Thus
$x^4+ax^3+bx^2+ax+1=(x^2+cx+1)(x^2+dx+1)$
If $x=1$ is to be a root then it must give $c=-2$ or $d=-2$ in one of the factors, as no other choice has the root $x=1$. Taking $c$ as $-2$ we then get the complex roots if $-2<d<2$, corresponding to a negative discriminate for the $x^2+dx+1$ factor.
Expanding the product $(x^2+cx+1)(x^2+dx+1)$ and equating this to $x^4+ax^3+bx^2+ax+1$ then implies $a=c+d=d-2$ so the solution to the problem is $-4<a<0$.
add a comment |
When a polynomial is "palindromic", reading the same way forwards and backwards, it must also have palindromic factors. Thus
$x^4+ax^3+bx^2+ax+1=(x^2+cx+1)(x^2+dx+1)$
If $x=1$ is to be a root then it must give $c=-2$ or $d=-2$ in one of the factors, as no other choice has the root $x=1$. Taking $c$ as $-2$ we then get the complex roots if $-2<d<2$, corresponding to a negative discriminate for the $x^2+dx+1$ factor.
Expanding the product $(x^2+cx+1)(x^2+dx+1)$ and equating this to $x^4+ax^3+bx^2+ax+1$ then implies $a=c+d=d-2$ so the solution to the problem is $-4<a<0$.
add a comment |
When a polynomial is "palindromic", reading the same way forwards and backwards, it must also have palindromic factors. Thus
$x^4+ax^3+bx^2+ax+1=(x^2+cx+1)(x^2+dx+1)$
If $x=1$ is to be a root then it must give $c=-2$ or $d=-2$ in one of the factors, as no other choice has the root $x=1$. Taking $c$ as $-2$ we then get the complex roots if $-2<d<2$, corresponding to a negative discriminate for the $x^2+dx+1$ factor.
Expanding the product $(x^2+cx+1)(x^2+dx+1)$ and equating this to $x^4+ax^3+bx^2+ax+1$ then implies $a=c+d=d-2$ so the solution to the problem is $-4<a<0$.
When a polynomial is "palindromic", reading the same way forwards and backwards, it must also have palindromic factors. Thus
$x^4+ax^3+bx^2+ax+1=(x^2+cx+1)(x^2+dx+1)$
If $x=1$ is to be a root then it must give $c=-2$ or $d=-2$ in one of the factors, as no other choice has the root $x=1$. Taking $c$ as $-2$ we then get the complex roots if $-2<d<2$, corresponding to a negative discriminate for the $x^2+dx+1$ factor.
Expanding the product $(x^2+cx+1)(x^2+dx+1)$ and equating this to $x^4+ax^3+bx^2+ax+1$ then implies $a=c+d=d-2$ so the solution to the problem is $-4<a<0$.
answered Nov 28 '18 at 2:04
Oscar Lanzi
12.1k12036
12.1k12036
add a comment |
add a comment |
Here's a different approach. (I can't say for sure whether you can or cannot do it by comparing coefficients though!)
For a degree 2 polynomial, we know that it has complex roots (assuming the coefficients are real) iff the discriminant is negative. There's a similar generalisation for high degree polynomial.
In particular, a degree 4 polynomial
$$ax^4+bx^3+cx^2+dx+e$$
has discriminant
$$256a^3e^3-192a^2bde^2-128a^2c^2e^2+144a^2d^2e...$$
(see https://en.wikipedia.org/wiki/Discriminant#Degree_4 for the full formula). And we know that since $x=1$ is a root there can only be a pair of complex roots so we require the discriminant to be negative.
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
add a comment |
Here's a different approach. (I can't say for sure whether you can or cannot do it by comparing coefficients though!)
For a degree 2 polynomial, we know that it has complex roots (assuming the coefficients are real) iff the discriminant is negative. There's a similar generalisation for high degree polynomial.
In particular, a degree 4 polynomial
$$ax^4+bx^3+cx^2+dx+e$$
has discriminant
$$256a^3e^3-192a^2bde^2-128a^2c^2e^2+144a^2d^2e...$$
(see https://en.wikipedia.org/wiki/Discriminant#Degree_4 for the full formula). And we know that since $x=1$ is a root there can only be a pair of complex roots so we require the discriminant to be negative.
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
add a comment |
Here's a different approach. (I can't say for sure whether you can or cannot do it by comparing coefficients though!)
For a degree 2 polynomial, we know that it has complex roots (assuming the coefficients are real) iff the discriminant is negative. There's a similar generalisation for high degree polynomial.
In particular, a degree 4 polynomial
$$ax^4+bx^3+cx^2+dx+e$$
has discriminant
$$256a^3e^3-192a^2bde^2-128a^2c^2e^2+144a^2d^2e...$$
(see https://en.wikipedia.org/wiki/Discriminant#Degree_4 for the full formula). And we know that since $x=1$ is a root there can only be a pair of complex roots so we require the discriminant to be negative.
Here's a different approach. (I can't say for sure whether you can or cannot do it by comparing coefficients though!)
For a degree 2 polynomial, we know that it has complex roots (assuming the coefficients are real) iff the discriminant is negative. There's a similar generalisation for high degree polynomial.
In particular, a degree 4 polynomial
$$ax^4+bx^3+cx^2+dx+e$$
has discriminant
$$256a^3e^3-192a^2bde^2-128a^2c^2e^2+144a^2d^2e...$$
(see https://en.wikipedia.org/wiki/Discriminant#Degree_4 for the full formula). And we know that since $x=1$ is a root there can only be a pair of complex roots so we require the discriminant to be negative.
answered Nov 28 '18 at 1:50
Rcwt
3871210
3871210
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
add a comment |
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
I doubt this approach is feasible as I will be solving sixth degree $p(a)< 0$.
– henceproved
Nov 28 '18 at 1:56
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
Actually your polynomial always has a double root so the discriminant is always 0 for all $a$ (from which you can work from there). But anyway the above answer is definitely better.
– Rcwt
Nov 28 '18 at 2:04
add a comment |
Considering that $$p(x) = x^4 + ax^3 + bx^2 + ax + 1$$ If $x=1$ is a root, then
$$p(1)=2 a+b+2=0 implies b=-2-2a$$ and
$$p(x)=(x-1),(x^3+(a+1) x^2-(a+1) x-1)$$ Clearly, $x=1$ is also a root for the second term making
$$p(x)=(x-1)^2,(x^2+(a+2) x+1)$$ So, there are complex roots if the discriminant $Delta$ of the remaining quadratic equation is negative.
add a comment |
Considering that $$p(x) = x^4 + ax^3 + bx^2 + ax + 1$$ If $x=1$ is a root, then
$$p(1)=2 a+b+2=0 implies b=-2-2a$$ and
$$p(x)=(x-1),(x^3+(a+1) x^2-(a+1) x-1)$$ Clearly, $x=1$ is also a root for the second term making
$$p(x)=(x-1)^2,(x^2+(a+2) x+1)$$ So, there are complex roots if the discriminant $Delta$ of the remaining quadratic equation is negative.
add a comment |
Considering that $$p(x) = x^4 + ax^3 + bx^2 + ax + 1$$ If $x=1$ is a root, then
$$p(1)=2 a+b+2=0 implies b=-2-2a$$ and
$$p(x)=(x-1),(x^3+(a+1) x^2-(a+1) x-1)$$ Clearly, $x=1$ is also a root for the second term making
$$p(x)=(x-1)^2,(x^2+(a+2) x+1)$$ So, there are complex roots if the discriminant $Delta$ of the remaining quadratic equation is negative.
Considering that $$p(x) = x^4 + ax^3 + bx^2 + ax + 1$$ If $x=1$ is a root, then
$$p(1)=2 a+b+2=0 implies b=-2-2a$$ and
$$p(x)=(x-1),(x^3+(a+1) x^2-(a+1) x-1)$$ Clearly, $x=1$ is also a root for the second term making
$$p(x)=(x-1)^2,(x^2+(a+2) x+1)$$ So, there are complex roots if the discriminant $Delta$ of the remaining quadratic equation is negative.
answered Nov 28 '18 at 3:53
Claude Leibovici
119k1157132
119k1157132
add a comment |
add a comment |
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