Checking irreducibility over \(\mathbb{R}\) - TIFR 2013 Sum 37

Let's discuss a problem based on Checking irreducibility over \(\mathbb{R}\) from TIFR 2013, Problem 37. Try this Problem and then read the solution.

Question: Checking irreducibility over \(\mathbb{R}\)

True/False?

The polynomial \(x^3+3x-2\pi \) is irreducible over \(\mathbb{R}\)

Hint:

When is a odd degree polynomial irreducible over \(\mathbb{R}\)?

Discussion:

A polynomial \(p(x)\) is irreducible over a field if it can not be written as product of two non-trivial polynomials having coefficients from the same field.

In other words, a polynomial \(p(x) \in \mathbb{F}[x] \) of degree \(k\) is irreducible over \(\mathbb{F}\) if there is no non-constant polynomial of degree \(<k\) dividing \(p(x)\) in \(\mathbb{F}[x]\).

For a polynomial \(p(x)\) of odd degree \( \ge 3 \) over \(\mathbb{R}\) there is always a real root \(\alpha\). This is because the complex roots always occur in conjugate pairs. Therefore, \((x-\alpha ) \) divides \(p(x)\).

Therefore, \(p(x)\) is not irreducible.

Some useful links:

• Number of irreducible polynomials
• Real Analysis YouTube Playlist