Problem #7
Let Mn denote the least common multiple of the integers
1, 2, 3, ..., n. For example, M1 = 1,
M2 = 2, M3 = 6, M4 =
12, M5 = 60, M6 = 60, ...
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Give a complete characterization of those n for which
Mn = Mn-1.
Source: Australian Mathematical Olympiad
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Can you find arbitrarily long stretches such that
Mn = Mn+1
= Mn+2 = ... ?