Idempotents (noch nicht übersetzt)

Problem 407

If we calculate a2 mod 6 for 0 ≤ a ≤ 5 we get: 0,1,4,3,4,1.

The largest value of a such that a2a mod 6 is 4.
Let's call M(n) the largest value of a < n such that a2a (mod n).
So M(6) = 4.

Find  M(n) for 1 ≤ n ≤ 107.