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Show that the number 1992 appears in the sequence

Source: IMO Shortlist 1992, Problem 14

August 13, 2008
inductionalgebrarecurrence relationSequenceIMO ShortlistIMO Longlist

Problem Statement

For any positive integer x x define g(x) g(x) as greatest odd divisor of x, x, and f(x) \equal{} \begin{cases} \frac {x}{2} \plus{} \frac {x}{g(x)} & \text{if \ $$x$$ is even}, \\ 2^{\frac {x \plus{} 1}{2}} & \text{if \ $$x$$ is odd}. \end{cases} Construct the sequence x_1 \equal{} 1, x_{n \plus{} 1} \equal{} f(x_n). Show that the number 1992 appears in this sequence, determine the least n n such that x_n \equal{} 1992, and determine whether n n is unique.