, 2016

Let $P = A_{1} A_{2} \ldots A_{k}$ and let $A_{k+i} = A_{i}$ for $i \geqslant 1$. By the Shoelace Formula, the area of any convex polygon with integral coordinates is half an integer. Therefore, $2 S$ is an integer. We shall prove by induction on $k \geqslant 3$ that $2 S$ is divisible by $n$. Clearly, it suffices to consider $n = p^{t}$ where $p$ is an odd prime and $t \geqslant 1$.

For the base case $k = 3$, let the side lengths of $P$ be $\sqrt{n a}$, $\sqrt{n b}$, $\sqrt{n c}$ where $a, b, c$ are positive integers. By Heron's Formula,
$$16 S^{2} = n^{2} \left(2 a b + 2 b c + 2 c a - a^{2} - b^{2} - c^{2}\right).$$
This shows $16 S^{2}$ is divisible by $n^{2}$. Since $n$ is odd, $2 S$ is divisible by $n$.

Assume $k \geqslant 4$. If the square of length of one of the diagonals is divisible by $n$, then that diagonal divides $P$ into two smaller polygons, to which the induction hypothesis applies. Hence we may assume that none of the squares of diagonal lengths is divisible by $n$. As usual, we denote by $\nu_{p}(r)$ the exponent of $p$ in the prime decomposition of $r$. We claim the following.

- Claim. $\nu_{p}\left(A_{1} A_{m}^{2}\right) > \nu_{p}\left(A_{1} A_{m+1}^{2}\right)$ for $2 \leqslant m \leqslant k-1$.

Proof. The case $m = 2$ is obvious since $\nu_{p}\left(A_{1} A_{2}^{2}\right) \geqslant p^{t} > \nu_{p}\left(A_{1} A_{3}^{2}\right)$ by the condition and the above assumption.

Suppose $\nu_{p}\left(A_{1} A_{2}^{2}\right) > \nu_{p}\left(A_{1} A_{3}^{2}\right) > \cdots > \nu_{p}\left(A_{1} A_{m}^{2}\right)$ where $3 \leqslant m \leqslant k-1$. For the induction step, we apply Ptolemy's Theorem to the cyclic quadrilateral $A_{1} A_{m-1} A_{m} A_{m+1}$ to get
$$A_{1} A_{m+1} \times A_{m-1} A_{m} + A_{1} A_{m-1} \times A_{m} A_{m+1} = A_{1} A_{m} \times A_{m-1} A_{m+1}$$
which can be rewritten as
$$\begin{align*} A_{1} A_{m+1}^{2} \times A_{m-1} A_{m}^{2} = & A_{1} A_{m-1}^{2} \times A_{m} A_{m+1}^{2} + A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2} \\ & - 2 A_{1} A_{m-1} \times A_{m} A_{m+1} \times A_{1} A_{m} \times A_{m-1} A_{m+1} \tag{1} \end{align*}$$
From this, $2 A_{1} A_{m-1} \times A_{m} A_{m+1} \times A_{1} A_{m} \times A_{m-1} A_{m+1}$ is an integer. We consider the component of $p$ of each term in (1). By the inductive hypothesis, we have $\nu_{p}\left(A_{1} A_{m-1}^{2}\right) > \nu_{p}\left(A_{1} A_{m}^{2}\right)$. Also, we have $\nu_{p}\left(A_{m} A_{m+1}^{2}\right) \geqslant p^{t} > \nu_{p}\left(A_{m-1} A_{m+1}^{2}\right)$. These give
$$\begin{equation*} \nu_{p}\left(A_{1} A_{m-1}^{2} \times A_{m} A_{m+1}^{2}\right) > \nu_{p}\left(A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right) \tag{2} \end{equation*}$$
Next, we have $\nu_{p}\left(4 A_{1} A_{m-1}^{2} \times A_{m} A_{m+1}^{2} \times A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right) = \nu_{p}\left(A_{1} A_{m-1}^{2} \times A_{m} A_{m+1}^{2}\right) + \nu_{p}\left(A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right) > 2 \nu_{p}\left(A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right)$ from (2). This implies
$$\begin{equation*} \nu_{p}\left(2 A_{1} A_{m-1} \times A_{m} A_{m+1} \times A_{1} A_{m} \times A_{m-1} A_{m+1}\right) > \nu_{p}\left(A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right) \tag{3} \end{equation*}$$
Combining (1), (2) and (3), we conclude that
$$\nu_{p}\left(A_{1} A_{m+1}^{2} \times A_{m-1} A_{m}^{2}\right) = \nu_{p}\left(A_{1} A_{m}^{2} \times A_{m-1} A_{m+1}^{2}\right)$$
By $\nu_{p}\left(A_{m-1} A_{m}^{2}\right) \geqslant p^{t} > \nu_{p}\left(A_{m-1} A_{m+1}^{2}\right)$, we get $\nu_{p}\left(A_{1} A_{m+1}^{2}\right) < \nu_{p}\left(A_{1} A_{m}^{2}\right)$. The Claim follows by induction.