[seqfan] Reversing A281684.
michel.marcus at free.fr
michel.marcus at free.fr
Thu Mar 4 16:34:02 CET 2021
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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