It often happens that a polynomial \(a(y)\), as it stands, does not satisfy the
conditions of Eisenstein's criterion, but with a simple change of variable
\(y=x+c\), it does. It is important to note that if \(a(x)\) can be factored into
\(p(x) q(x)\), then certainly \(a(x+c)\) can be factored into \(p(x+c) q(x+c)\).
Thus, the irreducibility of \(a(x+c)\) implies the irreducibility of \(a(x)\)
(a) Use the change of variable \(y=x+1\) to show that \(x^{4}+4 x+1\) is
irreducible in \(\mathbb{Q}[x]\). [In other words, test \((x+1)^{4}+4(x+1)+1\) by
Eisenstein's criterion.]
(b) Find an appropriate change of variable to prove that the following are
irreducible in \(\mathbb{[ x ]}\)
$$
x^{4}+2 x^{2}-1 ; \quad x^{3}-3 x+1 ; \quad x^{4}+1 ; \quad x^{4}-10 x^{2}+1
$$