Inspired by Satvik

Does there exist a power of 2 whose first 4 digits are identical?

Inspiration

Yes, for all digits 1 to 9 Yes, for some, but not all digits 1 to 9 No, never

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1 solution

Shourya Pandey
May 31, 2017

We show that for any number N N , there exists a power of 2 whose digits start with N N . Indeed, we want

N × 1 0 d < 2 m < ( N + 1 ) × 1 0 d N\times10^{d} < 2^m < (N+1)\times10^{d} , for some non-negative integer d d , or

d × l o g N < m × l o g 10 2 < d × l o g ( N + 1 ) d\times log N < m\times log_{10} 2 < d\times log(N+1) . Now l o g 2 log 2 is irrational, so there exist integers a , b a,b for which a b 2 < ϵ |a-b\sqrt{2}| < \epsilon for any positive ϵ \epsilon . Can you finish the proof from here?

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