Prove that at any point in time between 9 a.m. on the Monday of Week 1 and 6 p.m. on the Friday of Week 3, there are at least two players who would have completed the same number of games in the tournament till that point.
2
Step-by-Step Solution
Key idea: This is a Pigeonhole Principle application on a dynamic tournament state. Recognizable by "at any point in time" and "at least two players".
Step 1: Define the state at an arbitrary time . Let be the number of games completed by player at time , for .
Step 2: Identify the range of possible values for . Since each player plays exactly 19 games in total, .
Step 3: Analyze the boundary conditions. Can and coexist at the same time ?
If some player has completed 19 games, they must have played against every other player, including a player .
Therefore, must have completed at least 1 game (the one against ).
This means and cannot both be in the set of completed games at time .
Step 4: Apply the Pigeonhole Principle. The set of possible values for is either or .
In either case, there are at most 19 distinct possible values for the number of games completed.
Step 5: Conclude. We have 20 players (pigeons) and at most 19 possible values for completed games (pigeonholes).
By the Pigeonhole Principle, at least two players must have completed the same number of games at time .
Answer: 2