2001 USAMO Problems/Problem 1
Each of eight boxes contains six balls. Each ball has been colored with one of colors, such that no two balls in the same box are the same color, and no two colors occur together in more than one box. Determine, with justification, the smallest integer for which this is possible.
We claim that is the minimum. Consider the following construction (replacing colors with numbers) which fulfills this:
Suppose a configuration exists with .
Suppose a ball appears or more times. Then the remaining balls of the boxes must be distinct, so that there are at least balls, contradiction. If a ball appears or more times, the remaining balls of the boxes must be distinct, leading to balls. The fifth box can contain at most four balls from the previous boxes, and then the remaining two balls must be distinct, leading to , contradiction.
However, by the Pigeonhole Principle, at least one ball must appear times. Without loss of generality suppose that appears three times, and let the boxes that contain these have balls with colors . Each of the remaining five boxes can have at most balls from each of these boxes. Thus, each of the remaining five boxes must have additional balls . Thus, it is necessary that we use balls to fill a grid by the same rules.
Again, no balls may appear times, but by Pigeonhole, one ball must appear times. Without loss of generality, let this ball have color ; then the three boxes containing must have at least balls, contradiction.
Therefore, is the minimum.
Let be the number of balls which are the same color as the ball in box (including that ball). For a fixed box , , consider the sums For each fixed , since no pair of colors is repeated, each of the remaining seven boxes can contribute at most one ball to . Thus, . It follows by the convexity of that is minimized when one of the is equal to 3 and the other five equal to 2. Hence . Note that Hence there must be 23 colors. The construction for is above.
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