[seqfan] Re: when is 1234...n a prime?

Hans Havermann gladhobo at teksavvy.com
Thu Oct 1 02:57:02 CEST 2015


To underscore what an achievement Dana Jacobsen's non-find in the first 64000 terms represents: Unless the term had a relatively small factor, it would take me about three hours (in Mathematica) to test *one* of these numbers at the upper end. To do that many thousands represents (for me) *months* of computation. The closest I have come to doing something like this is looking for a prime in (10^n*78880-1)/3 where I believe n > 332700. Yes it would be pretty exciting to find (any) such large probable prime, not least because it would (likely) be in the top 100 (largest) known probable primes.

> On Sep 29, 2015, at 9:18 PM, Neil Sloane <njasloane at gmail.com> wrote:
> 
> Consider the sequence with nth term equal to the concatenation of the decimal numbers 1234...n (https://oeis.org/A007908). When is the first prime? The comments in A007908 say that there should be infinitely many primes, and that there are no primes among the first 64000 terms. If you would like to help with this search, you could leave a comment in A007908 saying that there are no primes among terms X through Y, or, of course, that n = Z gives a (probable) prime, which would be pretty exciting.


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