Define the function $\xi : \mathbb Z^2 \to \mathbb Z$ by $\xi(n,k) = 1$ when $n \le k$ and $\xi(n,k) = -1$ when $n > k$, and construct the polynomial \[ P(x_1, \dots, x_{1000}) = \prod_{n=1}^{1000} \left( \sum_{k=1}^{1000} \xi(n,k)x_k \right). \]
(a) Determine the coefficient of $x_1x_2 \dots x_{1000}$ in $P$.
(b) Show that if $x_1, x_2, \dots, x_{1000} \in \left\{ -1,1 \right\}$ then $P(x_1,x_2,\dots,x_{1000}) = 0$.

[i]Proposed by Evan Chen[/i]
### Part (a)
1. Define the function $\xi : \mathbb{Z}^2 \to \mathbb{Z}$ by:
   \[
   \xi(n,k) = 
   \begin{cases} 
   1 & \text{if } n \le k \\
   -1 & \text{if } n > k 
   \end{cases}
   \]
2. Construct the polynomial:
   \[
   P(x_1, \dots, x_{1000}) = \prod_{n=1}^{1000} \left( \sum_{k=1}^{1000} \xi(n,k)x_k \right)
   \]
3. To find the coefficient of $x_1 x_2 \dots x_{1000}$ in $P$, consider the term:
   \[
   \prod_{n=1}^{1000} \left( \sum_{k=1}^{1000} \xi(n,k)x_k \right)
   \]
4. For each $n$, the sum $\sum_{k=1}^{1000} \xi(n,k)x_k$ contains $1001-n$ terms where $\xi(n,k) = 1$ and $n-1$ terms where $\xi(n,k) = -1$.
5. The coefficient of $x_1 x_2 \dots x_{1000}$ is obtained by selecting $x_k$ from each sum $\sum_{k=1}^{1000} \xi(n,k)x_k$ such that the product of the coefficients of $x_k$ is non-zero.
6. However, due to the symmetry and the alternating signs, the product of the coefficients for $x_1 x_2 \dots x_{1000}$ will always cancel out to zero.

Thus, the coefficient of $x_1 x_2 \dots x_{1000}$ in $P$ is:
\[
\boxed{0}
\]

### Part (b)
1. Consider the polynomial $P(x_1, x_2, \dots, x_{1000})$ where $x_i \in \{-1, 1\}$.
2. Define the sum $S_n = \sum_{k=1}^n x_k - \sum_{k=n+1}^{1000} x_k$.
3. Note that $S_{1000} = x_1 + x_2 + \dots + x_{1000}$.
4. We need to show that $P(x_1, x_2, \dots, x_{1000}) = 0$ by proving that at least one of the terms $\sum_{k=1}^{1000} \xi(n,k)x_k$ is zero.
5. Assume by contradiction that none of these terms is zero.
6. Observe that:
   \[
   S_{n+1} = S_n - 2x_{n+1}
   \]
   Therefore:
   \[
   |S_n - S_{n+1}| = |2x_{n+1}| = 2
   \]
7. Since $x_i \in \{-1, 1\}$, $S_n$ changes by 2 for each $n$.
8. Note that $S_{1000} = -S_1 + 2x_1$.
9. If $S_1 \neq 0$, then $S_1$ must be even because it is the sum of an even number of terms.
10. Assume $S_1 > 0$, then $S_1 \geq 2$.
11. Since $S_{1000} = -S_1 + 2x_1$, and $x_1 \in \{-1, 1\}$, $S_{1000}$ must be negative if $S_1 \geq 2$.
12. This implies that $S_{1000} < 0$, which contradicts the assumption that none of the sums is zero.
13. Therefore, at least one of the sums $\sum_{k=1}^{1000} \xi(n,k)x_k$ must be zero, making $P(x_1, x_2, \dots, x_{1000}) = 0$.

\[
\blacksquare
\]