## Applications of Complex Number Analysis to Divisibility Problems

Prove that ${(x+y)}^n-x^n-y^n$ is divisible by $xy(x+y) \times (x^2+xy+y^2)$ if $n$ is an odd number not divisible by $3$ . Prove that ${(x+y)}^n-x^n-y^n$ is divisible by $xy(x+y) \times {(x^2+xy+y^2)}^2$ if $n \equiv \pmod{6}1$ Solution 1.Considering the given expression as a polynomial in $y$ , let us put $y=0$ . We see that at $y=0$ the polynomial vanishes (for any $x$ ). Therefore our polynomial is divisible by…