Problem 2

Question

Prove that $x^{4}+10 x^{3}+7$ is irreducible in $\mathbb{Q}[x]$ by using the natural homomorphism from $\mathbb{Z}$ to $\mathbb{Z}_{5}$.

Step-by-Step Solution

Verified

Answer

The polynomial $ x^{4} + 10x^{3} + 7 $ is irreducible over $ \mathbb{Q} $.

1Step 1: Identify the Problem

We want to show that the polynomial $ f(x) = x^{4} + 10x^{3} + 7 $ is irreducible over $ \mathbb{Q} $. To do this, we will use a homomorphism from $ \mathbb{Z} $ to $ \mathbb{Z}_{5} $ and check for irreducibility modulo 5.

2Step 2: Apply Natural Homomorphism

Consider the ring homomorphism from $ \mathbb{Z} $ to $ \mathbb{Z}_{5} $. Applying this to the polynomial gives us $ f(x) \equiv x^4 + 0x^3 + 7 \equiv x^4 + 2 \pmod{5} $.

3Step 3: Check Irreducibility Modulo 5

To show $ x^4 + 2 $ is irreducible over $ \mathbb{Z}_{5} $, we need to check whether it can be factored into polynomials of lower degree in this field. Test for roots in $ \mathbb{Z}_{5} $, i.e., check $ f(a) = 0 $ for $ a = 0, 1, 2, 3, 4 $.

4Step 4: Evaluate Polynomial at Roots

Substitute these values into $ f(a) = a^4 + 2 $. We find: $ f(0) = 2 $, $ f(1) = 1^4 + 2 = 3 $, $ f(2) = 2^4 + 2 = 18 \equiv 3 \pmod{5} $, $ f(3) = 3^4 + 2 = 83 \equiv 3 \pmod{5} $, and $ f(4) = 4^4 + 2 = 258 \equiv 3 \pmod{5} $. Since $ f(a) eq 0 $ for all $ a $ in $ \mathbb{Z}_{5} $, there are no roots.

5Step 5: Conclude Irreducibility

Since $ x^4 + 2 $ has no linear factors in $ \mathbb{Z}_{5} $, it cannot be factored into quadratic factors either (as no quadratic polynomial with coefficients in $ \mathbb{Z}_{5} $ would produce just the constant term 2 when squared). Thus $ x^4 + 2 $ is irreducible over $ \mathbb{Z}_{5} $.

6Step 6: Final Conclusion for $ \mathbb{Q} $

By Eisenstein's Criterion, if a polynomial is irreducible over $ \mathbb{Z}_p $, it is also irreducible over $ \mathbb{Q} $. Hence, $ x^4 + 10x^3 + 7 $ is irreducible in $ \mathbb{Q}[x] $.

Key Concepts

Homomorphism from $ \mathbb{Z} $ to $ \mathbb{Z}_p $Checking Roots Modulo PrimeIrreducibility Criteria

Homomorphism from $ \mathbb{Z} $ to $ \mathbb{Z}_p $

A homomorphism is a mathematical concept that maps one algebraic structure to another in a way that preserves operations. For the problem at hand, we use a specific type of homomorphism: the natural homomorphism from the integers $ \mathbb{Z} $ to the modular arithmetic ring $ \mathbb{Z}_p $. This means we're taking the coefficients of a polynomial based in $ \mathbb{Z} $ and considering them in the context of a ring where arithmetic is performed modulo $ p $, a prime number.
This natural homomorphism is particularly helpful when studying the properties of polynomials because reducing coefficients modulo a prime often simplifies the problem without altering the critical properties we've set out to examine. In the exercise, we map the polynomial $ f(x) = x^{4} + 10x^{3} + 7 $ to $ \mathbb{Z}_{5} $, the field of integers modulo 5.

This results in simplifying the polynomial to $ x^4 + 2 $ under modulo 5, as coefficients like 10 become 0 when considered modulo 5, since 10 itself is a multiple of 5.
By transforming it into a simpler polynomial, this method allows us to check for irreducibility in a more manageable space.

Using homomorphisms is a powerful tool in algebra, helping us leverage modular arithmetic to reveal polynomial characteristics which can be hard to determine in the larger field of rational numbers.

Checking Roots Modulo Prime

One of the crucial steps in testing polynomial irreducibility in a finite field $ \mathbb{Z}_p $ is to check whether the polynomial has any roots. A root of a polynomial is simply a solution to the equation formed when the polynomial is set equal to zero.
In our problem, we are checking if the transformed polynomial $ x^4 + 2 $ has any roots in $ \mathbb{Z}_{5} $. This is done by evaluating the polynomial at each element of $ \mathbb{Z}_{5} $, which consists of the set \{0, 1, 2, 3, 4\}.

For example, substituting $ x = 0 $ in the polynomial gives $ 0^4 + 2 \equiv 2 \mod{5} $, indicating no root is found.
Continuing this process for $ x = 1, 2, 3, 4 $, we find out that in each case, the resultant is not zero under modulo 5 arithmetic.

Since none of these substitutions result in the polynomial equaling zero, $ x^4 + 2 $ has no roots within $ \mathbb{Z}_{5} $. This absence of roots implies that the polynomial cannot be factored into linear factors in $ \mathbb{Z}_{5} $, a key indicator of its irreducibility in this context.

Irreducibility Criteria

Irreducibility criteria help determine whether a polynomial cannot be factored into lower-degree polynomials with coefficients in the same field. One of the methods to prove irreducibility is to show **root exclusion**, as done in the prior section, where none of the modular equation results are zero.

Beyond finding roots, another approach is checking whether a polynomial can be decomposed into factors of lower degree, such as linear or quadratic polynomials. Since we established that $ x^4 + 2 $ has no roots in $ \mathbb{Z}_{5} $, it cannot have linear factors. Next, consider quadratic factors; none can multiply to a quartic polynomial with a constant term of 2 when using coefficients from $ \mathbb{Z}_{5} $.

In our exercise, this dual exclusion of both applicable factor types solidifies that the polynomial is irreducible over $ \mathbb{Z}_{5} $.
The connection was then drawn to rational numbers $ \mathbb{Q} $ using a criterion like Eisenstein's, which dictates similar characteristics of irreducibility over larger fields when observed in these finite fields.

This reinforces that the original polynomial $ x^{4} + 10x^{3} + 7 $ is irreducible over $ \mathbb{Q}[x] $. These criteria are central to understanding polynomial behavior and reveal powerful insights into their inherent structure.

Problem 1

Problem 2