Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Prove it United States

Problem:
In how many ways can the cells of a 4×44 \times 4 table be filled in with the digits 1,2,,91,2, \ldots, 9 so that each of the 4-digit numbers formed by the columns is divisible by each of the 4-digit numbers formed by the rows?

Solution

Solution:
If aa and bb are 4-digit numbers with the same first digit, and aa divides bb, then since b<a+10002ab < a + 1000 \leq 2a, bb must equal aa. In particular, since the number formed by the first row of the table divides the number in the first column (and both have the same first digit), these numbers must be equal; call their common value nn. Then, for k=2,3k = 2, 3, or 44, we find that the number in the kkth column and the number in the kkth row have the same first digit (namely the kkth digit of nn), so by the same reasoning, they are equal. Also, the smallest number bb formed by any column is divisible by the largest number aa formed by any row, but by the symmetry just proven, aa is also the largest number formed by any column, so aba \geq b. Since bb is divisible by aa, we must have equality. Then all columns contain the same number - and hence all rows also contain the same number - which is only possible if all 16 cells contain the same digit. Conversely, for each d=1,,9d = 1, \ldots, 9, filling in all 16 cells with the digit dd clearly gives a table meeting the required condition, so we have exactly 9 such tables, one for each digit.

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