Solving Congruences with Polynomials: A Prime Challenge

In summary, the polynomial $p(x) = (x^2 +1)(x^2 +2)(x^2 -2)$ has no rational roots, but satisfies the conditions of the statement if $x^2 +1,x^2 +2,x^2 -2 all have quadratic non-residues mod $p$.
  • #1
Fantini
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I'm having trouble with the following question:

Construct a polynomial $q(x) \neq 0$ with integer coefficients which has no rational roots but is such that for any prime $p$ we can solve the congruence $q(x) \equiv 0 \mod p$ in the integers.

Any hints on how to even start the problem will be strongly appreciated. (Nod) Thanks.
 
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  • #2
Fantini said:
I'm having trouble with the following question:

Construct a polynomial $q(x) \neq 0$ with integer coefficients which has no rational roots but is such that for any prime $p$ we can solve the congruence $q(x) \equiv 0 \mod p$ in the integers.

Any hints on how to even start the problem will be strongly appreciated. (Nod) Thanks.

Hi Fantini, :)

Let me give you a hint. Consider the Fermat's little theorem and see whether you can think of a polynomial satisfying the given conditions.

Kind Regards,
Sudharaka.
 
  • #3
Thank you Sudharaka, but now I have found the solution. :) I will post it later.

Cheers.
 
  • #4
Fantini said:
Thank you Sudharaka, but now I have found the solution. :) I will post it later.

Cheers.

You are welcome. :) I was thinking about the polynomial \(q(x)=x^p-x\) so that the congruence \(q(x)\equiv 0(\mbox{mod } p)\) could be solved in the integers (by the Fermat's little theorem), but alas \(q\) has rational roots (examples are \(x=0,1\)). So to make it rational root less, we can do a little improvement. Take,

\[q(x)=x^p-x+p\]

By the Rational root theorem the list of possible rational roots that this polynomial could have are \(\pm 1,\pm p\). Clearly, \(x= \pm 1, p\) do not satisfy the equation \(q(x)=0\). Let \(x=-p\) be a root of \(q(x)\). Then,

\[q(-p)=0\Rightarrow (-p)^p+2p=0\]

If \(p\) is even, \(p=2\) and we have \(p^p+2p=8=0\) which is contradictory. If \(p\) is odd,

\[\Rightarrow -p^p+2p=0\Rightarrow p^{p-1}=2\]

Since \(p\) is an odd prime, \(p\geq 3\). Hence the equation \(p^{p-1}=2\) cannot be satisfied. Therefore we see that there are no rational roots for,

\[q(x)=x^p-x+p\]

Interested to see your solution. :)
 
  • #5
A quick internet search comes up with the polynomial $p(x) = (x^2-2)(x^2-3)(x^2-6)$ (see here, for example).

The reason that works is that if 2 and 3 are both quadratic non-residues mod $p$ then their product 6 will be a quadratic residue. On the other hand, $p(x)$ clearly has no rational roots.
 
  • #6
The solution is similar to what Opalg's search found. We considered the polynomial $q(x) = (x^2 +1)(x^2 +2)(x^2 -2)$ and, using elementary number theory arguments such as Legendre's symbol and quadratic residues, proved it satisfied the conditions of the statement. :D

Thanks for the search link, Opalg!
 

FAQ: Solving Congruences with Polynomials: A Prime Challenge

What is the definition of a congruence?

A congruence is a mathematical relation that indicates two numbers or objects are equivalent in some way. In the context of polynomials, congruences refer to the relationship between two polynomials that are equal when evaluated at certain values.

How do you solve a congruence with polynomials?

To solve a congruence with polynomials, you need to use the methods of polynomial division and modular arithmetic. First, divide the polynomial on both sides of the congruence by the polynomial that is being equated to zero. Then, use modular arithmetic to find the roots of the resulting polynomial.

What is the Prime Challenge in solving congruences with polynomials?

The Prime Challenge in solving congruences with polynomials is to find the solutions to the congruence when the modulus is a prime number. This requires using advanced techniques such as modular inverse and the Chinese Remainder Theorem.

Why is solving congruences with polynomials important?

Solving congruences with polynomials is important in various areas of mathematics, including number theory, abstract algebra, and cryptography. It also has practical applications in fields such as computer science and engineering.

What are some common mistakes to avoid when solving congruences with polynomials?

Some common mistakes to avoid when solving congruences with polynomials include forgetting to divide both sides by the polynomial being equated to zero, not using the correct modulus, and making errors when applying modular arithmetic rules. It is also important to check for extraneous solutions and to simplify the resulting polynomial before finding the roots.

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