[seqfan] Re: Primes of the form (4^p+1)/5^t

israel at math.ubc.ca israel at math.ubc.ca
Wed Mar 15 07:31:54 CET 2017

That may tell you something about t, but what does it have to do with 
whether N is prime?

On Mar 14 2017, Don Reble wrote:

>> Let N=(4^p+1)/5^t, where p is prime, 5^t is the most power of 5 dividing
>> 4^p+1. For p=2,3,5, N=17,13,41. What is the next prime p for which N is
>> prime?
>    To prove there aren't any more prime N's, a first step is to
>    show that if 5^n divides either of (2^(2a+1) +- 2^(a+1) + 1),
>    then 5^(n-1) divides (2a+1). Calculations suggest it's true,
>    but I'm stuck.

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