Question
If is a finite field show that there is always an irreducible polynomial of the form x3 - x + a where
. (Hint: Show that
is not a surjective map.)
Answer :
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Consider the map (f: \mathbb{F} \to \mathbb{F}) defined by (f(x) = x^3 - x). We want to show that this map is not surjective, which will imply that there exists some (a \in \mathbb{F}) such that the polynomial (x^3 - x + a) has no root in (\mathbb{F}) and is therefore irreducible over (\mathbb{F}). Let (|\mathbb{F}| = q). The map (f(x) = x^3 - x = x(x-1)(x+1)) (factorization works in characteristic not 2; if (\mathrm{char}(\mathbb{F}) = 2), then (x^3 - x = x^3 + x = x(x^2 + 1))). In either case, consider the number of solutions of (f(x) = 0): 1. In characteristic (\neq 2),
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