Divisibility involving root of unity Let $p$ be a prime number and $\omega$ be a $p$-th root of unity. Suppose $a_0,a_1, \dots, a_{p-1}, b_0, b_1, \dots, b_{p-1}$ be integers such that $a_0 \omega^0+a_1 \omega^1+ \dots a_{p-1} \omega^{p-1}$ and $b_0 \omega^0 + b_1 \omega^1 + \dots b_{p-1} \omega^{p-1}$ are also integers
Prove that $(a_0 \omega^0+a_1 \omega^1+ \dots a_{p-1} \omega^{p-1})-(b_0 \omega^0 + b_1 \omega^1 + \dots b_{p-1} \omega^{p-1})$ is divisible by $p$ if and only if $p$ divides all of $a_0-b_0$, $a_1-b_1$, $\dots$, $a_{p-1}-b_{p-1}$
 A: Irreducible polynomial over $\mathbb Z[X]$
Consider the factorization $X^p-1=(X-1)\Phi_p$ where $\Phi_p$ is the cyclotomic polynomial defined to contain all primitive $p$'th roots of unity over $\mathbb C$. Dividing both sides by $(X-1)$ we get
$$
\Phi_p=X^{p-1}+X^{p-2}+...+1
$$
Since all cyclotomic polynomials are irreducible over $\mathbb Q[X]$ they are in particular irreducible over $\mathbb Z[X]$. So given $\omega\neq 1$ that is a $p$'th root of unity we then know that $\omega$ is a root of $\Phi_p$ and that if $\omega$ is a root of some other polynomial $f\in\mathbb Z[X]$ then $\Phi_p$ divides $f$.
The coefficients of each subexpression
Now if $a_0\omega^0+a_1\omega^1+...+a_{p-1}\omega^{p-1}=k$ is an integer then $\omega$ is a root of
$$
f:=(a_0-k)+a_1 X+...+a_{p-1} X^{p-1}\in\mathbb Z[X]
$$
so $\Phi_p$ divides $f$ in $\mathbb Z[X]$. Hence there must exist $s\in\mathbb Z$ so that $f=s\cdot\Phi_p$ showing that
$$
a_0-k=a_1=...=a_{p-1}=s
$$
A similar argument shows that we must have some $t\in\mathbb Z$ so that $b_0-m=b_1=...=b_{p-1}=t$ in order to have $b_0\omega^0+b_1\omega^1+...+a_{p-1}\omega^{p-1}=m\in\mathbb Z$.
The coefficients of the difference
With the above we see that
$$
\begin{align}
&(a_0\omega^0+a_1\omega^1+...+a_{p-1}\omega^{p-1})-(b_0\omega^0+b_1\omega^1+...+a_{p-1}\omega^{p-1})\\
&=(s+k-(t+m))\omega^0+(s-t)\omega^1+...+(s-t)\omega^{p-1}\\
&=k-m
\end{align}
$$
but this actually suggests that your statement we are trying to prove is wrong in the first place for this holds regardles of $s$ and $t$ so that you can always choose $k-m$ divisible by $p$ without having $a_i-b_i\equiv s-t=0$ mod $p$. This seems to me a contradiction to the statement we are trying to prove! Please correct me if I am mistaken.
