[seqfan] Re: Comment in A095810

Peter Munn techsubs at pearceneptune.co.uk
Mon Oct 30 20:46:20 CET 2017


Hi David,

I think it would be better expressed by changing "find" to "can
determine". So, it doesn't mean that 6 is a power of 2. Instead, it means
that you can't determine that a number ending in 6 is not a power of 2.
(Whereas you can determine that a number ending in 7 is not a power of 2.)

Best Regards,

Peter

On Mon, October 30, 2017 4:39 pm, David Corneth wrote:
> Hi all,
>
> There is a comment written in 2004 by Francisco Salinas in A095810
> <https://oeis.org/A095810> that might need some more editing I think.
> It's
> "If we have only the last k digits of a large integer N, you find that N
> is
> not a power of 2 if and only if the number given by those digits is
> divisible by 5 OR not a multiple of 2^k OR a value > 2^k*(5^k-1)."
>
> I think it would mean that 6 is a power of 2; it's a multiple of 2 and not
> divisible by 5 (The last test doesn't change the verdict as 10^k has k+1
> digits. 10^k - 2^k < N < 10^k isn't divisible by 2^k hence is excluded by
> the first test.).
>
> So, what to do with the comment?
>
> Best,
> David
>
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