Friday, 7 November 2014

combinatorics - Prove that for any given positive integer n,there exists a number having digits 0,1 which is divisible by n.




Prove that for any given positive integer n,there exists a number having digits 0,1 which is divisible by n.





Let that number be of the general form: x=¯bkbk1...b1b0 where bi{0,1}.How can we construct x to be divisible by the given n?


Answer



Let ak=111...11k=10k19



So, among a1,a2,...an+1 two must have the same remainder modulo n, say ai and aj and i>j. Thus naiaj=10i10j9=10j10ij19=111...11ij000...00j


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