Tuesday, 24 September 2013

sequences and series - If an+1=lfloor1.05timesanrfloor, does there exist N such that aNequiv0(mod10)?

I've known the following theorem.




Theorem: Supposing that an+1=1.05×an for any natural number n, there exists N such that aN0 (mod 10) for any integer 20a1100.



Proof: {an} is a monotonic increase sequence, so let's observe the minimum n such that an100 for any a1. The observation shows you that you'll always get any one of 100,101,102,103. Then, you get 101106111116121127133139145152159166174182191200



102107112117122128134140



103108113118123129135141148155162170,

so the proof is completed.



Then, here are my questions.




As far as I know, the next question still remains unsolved.



Question1: Supposing that an+1=1.05×an for any natural number n, does there exist N such that aN0 (mod 10) for any integer a120 ?



It is likely that such N would exist, but I'm facing difficulty.
I'm also interested in the following generalization.



Question2: Supposing that α>1 is a real number and that an+1=α×an for any natural number n, does there exist N such that aN0 (mod 10) for any integer a11α1 ?




Any help would be appreciated.

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