[seqfan] Re: Reversing A281684.

[Sean A. Irvine]

f(1), f(2), f(4) all exceed 800 if they exist.  My prime checking gets very
slow for bigger numbers.



On Fri, 5 Mar 2021 at 04:34, <michel.marcus at free.fr> wrote:

>
> Dear Seqfans
>
>
> A281684 is defined as: Least composite k such that the concatenation of n
> consecutive composites, starting from k, is a prime.
> It has a few small values: A281684(2)= 8; A281684(6)= 14; A281684(14)= 15;
> A281684(24)= 18; A281684(302)= 16.
>
>
> So is it possible to reverse it ?
> f(n) = least number k such that the concatenation of k composite numbers
> starting from the n-th composite is prime
>
>
> f(3) = 2 2nd composite : 8 ; 89 is prime
> f(5) = 646
> f(7) = 6 7th composite : 14 ; 141516182021 is prime
> f(8) = 14
> f(9) = 302
> f(10) = 24
>
>
> Is it possible to find f(1), f(2), f(4), f(6) that are the first missing
> values for f() ?
>
>
> Best
> MM
>
>
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