On stable polynomials of degrees $2,3,4$

Abstract: Let $q$ be a prime power. We construct stable polynomials of the form $b<sup>{m-1}(x+a)<sup>m+c(x+a)+d$ over a finite field $\mathbb{F}<em>{q}$ for $m=2,3,4$ by Capelli's lemma. When $m=3$ and $q$ is even, we confirm the conjecture of Ahmadi and Monsef-Shokri [2] that the polynomial $f(x) = x³ + x² + 1$ is stable over $\mathbb{F}</em>{2}$. Moreover, when $m=2$ and $q\equiv 1\pmod{4}$, we improve a lower bound of the number of quadratic stable polynomials by Gom\'ez-P\'erez and Nicol\'as [4].