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2008 appears on blackboard

Source: All Russian 2008, Grade 9, Problem 7

June 13, 2008
number theorygreatest common divisorrelatively primecombinatorics

Problem Statement

A natural number is written on the blackboard. Whenever number x x is written, one can write any of the numbers 2x \plus{} 1 and \frac {x}{x \plus{} 2}. At some moment the number 2008 2008 appears on the blackboard. Show that it was there from the very beginning.