Problem 1426

Let's look at the problem using graph theory. We can represent the tournament as a directed graph where each player is a vertex, and there is a directed edge from vertex $A$ to vertex $B$ if player $A$ beats player $B$. We need to prove that the maximal oriented path contains all the vertices.

1. Assume the contrary: Suppose the maximal oriented path does not contain all the vertices. Then, we can divide the set of vertices into two disjoint sets, $U$ and $V$, where $U$ contains the vertices from the maximal path and $V$ contains the others. Denote the path as $A_1 \rightarrow A_2 \rightarrow \ldots \rightarrow A_k$.

2. **Properties of vertices in $V$**: Every player in $V$ either loses or draws with $A_1$ and beats or draws with $A_k$ due to the maximality of the path.

3. No player can have two draws: Suppose a player $A$ draws with both $B$ and $C$. If $B$ defeats $C$, then for the drawing pair $A, C$, we have a vertex $B$ who does not lose to either $A$ or $C$, which is impossible. Therefore, a player cannot have two draws.

4. **No draws within $V$**: Suppose $X, Y \in V$ draw. Then $A_1$ must lose to one of them, and we can find a longer path $V \rightarrow A_1 \rightarrow \ldots \rightarrow A_k$, contradicting the maximality of the path.

5. Constructing the path: Start from $A_k$. Check if $A_k$ beats someone from $V$. If not, move to $A_{k-1}$ and so on. If $A_i$ beats someone from $V$, say $X$, then $X$ cannot have drawn with $A_{i+1}$ (since $i < k$) for obvious reasons. Therefore, $X$ must have won against $A_{i+1}$, allowing us to extend our path, which is impossible.

6. **Reaching $A_1$**: If we never stop, we reach $A_1$ and find that nobody from $U$ won over someone from $V$. Since $A_1$ cannot lose to someone from $V$, $A_1$ must draw with everyone from $V$. However, $A_1$ can have at most one draw, so $V$ has exactly one vertex, $Y$.

7. **Analyzing $Y$**: $Y$ cannot draw with $A_k$ since no player can have two draws. $Y$ cannot lose to $A_k$ because that would allow us to find a longer path. Therefore, $A_k$ must lose to $Y$.

8. **Draw within $U$**: If there is a draw within $U$, say between $L$ and $M$, then one of $L$ or $M$ must defeat $V$ (since $V$ cannot take any more draws), which is impossible because we are in case 2.

Thus, our assumption that the maximal oriented path does not contain all the vertices is wrong, and we are done.

$\blacksquare$