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## 20125Re: [PrimeNumbers] Re: 2^m+3^n and 2^n+3^m

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• Apr 18, 2009
--- On Fri, 4/17/09, David Broadhurst <d.broadhurst@...> wrote:
> Submitted to OEIS:
>
> Numbers n such that 2^x + 3^y is never prime when max(x,y) = n
>
> 1679, 1743, 4980, 4982, 5314, 5513, 5695, 6100, 6578,
> 7251, 7406, 7642, 8218, 8331, 9475, 9578, 9749
>
> Mark Underwood found that for each non-negative integer n < 1421
> there is at least one prime of the form 2^m + 3^n or 2^n + 3^m
> with m not exceeding n.
>
> This sequence consists of numbers for which there is no
> such prime.
>
> David Broadhurst estimated that a fraction in excess of 1/800
> of the natural numbers belongs to this sequence and found
> 17 instances with n < 10^4.
>
> For each of the remaining 9983 non-negative integers n < 10^4,
> a prime or probable prime of the form 2^x + 3^y was found with
> max(x,y) = n.
>
> Each probable prime was subjected to a combination of
> strong Fermat and strong Lucas tests.
>