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