IL-GILLIS 2020 Problem 4
At the start of the day, the four numbers were written on the board. Every minute, Danny replaces the four numbers written on the board with new ones according to the following rule: if the numbers written on the board are , then Danny first calculates the numbers \begin{align*} a'&=a+4b+16c+64d b'&=b+4c+16d+64a c'&=c+4d+16a+64b d'&=d+4a+16b+64c \end{align*} and replaces the numbers with the numbers . For which initial quadruples , will Danny write at some point a quadruple of numbers all of which are divisible by ?
Dataset text AI-MO AoPS. Quoted with source.
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