Question 88
For any positive integer $n$ set
$$
A_{n} = \{ j : 1 \leq j \leq n,\ (j, n) = 1 \}
$$
Find all $n$ such that the polynomial
$$
P_{n}(x) = \sum_{j \in A_{n}} x^{j-1}
$$
is irreducible over $\mathbb{Z}[x]$.
$$
A_{n} = \{ j : 1 \leq j \leq n,\ (j, n) = 1 \}
$$
Find all $n$ such that the polynomial
$$
P_{n}(x) = \sum_{j \in A_{n}} x^{j-1}
$$
is irreducible over $\mathbb{Z}[x]$.