A positive proper divisor is a positive divisor of a number, excluding itself. For positive integers n≥2, let f(n) denote the number that is one more than the largest proper divisor of n. Determine all positive integers n such that f(f(n)) = 2.
It's true for all prime n, because f(n) = 2 and f(2) = 2.
It's also true for any n = 2(p-1) where p is any prime, because then f(n) = n/2 + 1 = p, so f(f(n)) = 2.
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Posted by tomarken
on 2021-05-13 07:47:06 |
Solution
