<s>[INST] Factorization of $x^n + y^n$, what sort of coefficients show up? We know that$$a^2 + b^2 = (a + bi)(a - bi).$$ What are the complete factorizations of $a^3 + b^3$, $a^4 + b^4$, $\ldots$ , $a^k + b^k$, etc.? What sort of coefficients show up?
 [/INST] You could always write $a^n+b^n = a^n - c^n$ where $c = \xi_n b$, where $\xi_n = e^{\pi i/n}$ (so that $\xi_n^n = -1$). Then fixing $a$, the roots of $a^n - c^n$ are solutions to $a^n = c^n$ and so there are $n$ solutions for $c$; namely, the numbers in the form of $\zeta_n^k a$ where $0\leq k\leq n-1$ and $\zeta = e^{2\pi i/n}$ so that $\zeta_n^n = 1$. So the original polynomial factors completely over $\Bbb{C}$ as 
$$(c - a)(c - \zeta_na)(c - \zeta_n^2a)...(c - \zeta_n^{n-1}a)$$
Such $\zeta_n$ is called an $n^{th}$ root of unity. There are always $n$ $n^{th}$ roots of unity, namely, the distinct powers of $\zeta_n$. 
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