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NZ-NZMO 2022 Problem 5

  • Answer

The sequence x1,x2,x3,…x_{1},x_{2},x_{3},\ldots is defined by x1=2022x_{1} = 2022 and xn+1=7xn+5x_{n + 1} = 7x_{n} + 5 for all positive integers nn . Determine the maximum positive integer mm such that

xn(xn−1)(xn−2)…(xn−m+1)m!{\frac{x_{n}(x_{n}-1)(x_{n}-2)\ldots(x_{n}-m+1)}{m!}}

is never a multiple of 7 for any positive integer nn .

Dataset text NuminaMath olympiads. Quoted with source.

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