[seqfan] Re: Reversing A281684.

[Jack Grahl]

I think this is incorrect. a(1) gives the number of concatenated values in
a number starting 4689... which is prime.

On Thu, 4 Mar 2021, 18:54 Ami Eldar, <amiram.eldar at gmail.com> wrote:

> f(1) is easy: it cannot exist since the concatenation of 1 composite number
> cannot be prime.
>
> On Thu, Mar 4, 2021 at 5:34 PM <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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> >
>
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