2023 AIME II Problem 10
Attempt Problem 10 of the 2023 AIME II below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2023 AIME II solutions, or check the answer key.
All problems are used with official legal permission of the Mathematical Association of America (MAA).
10.
Let be the number of ways to place the integers through in the cells of a grid so that for any two cells sharing a side, the difference between the numbers in those cells is not divisible by One way to do this is shown below. Find the number of positive integer divisors of
Answer: 144
Small Hint:
Cells sharing a side must have different residues mod count the residue patterns first, then multiply by to place the actual numbers
Big Hint:
Each column shows two distinct residues; from any column, each of the three unordered residue pairs can come next in exactly one orientation, and each pair must occur twice
Solution:
The condition says adjacent cells have different residues mod Each residue class among has exactly members, so where is the number of ways to fill the grid with residues each used times, with adjacent cells different.
A column is an ordered pair of distinct residues. If the current column is and is the third residue, the next column must be one of each of the three unordered pairs occurs in exactly one allowed orientation. So a residue pattern is determined by the sequence of six unordered pairs together with the orientation of the first column. Since each residue must appear times, each of the three pairs must be used exactly twice, giving sequences and
Therefore which has positive divisors.
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