Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #17 Feb 22 2021 21:14:36
%S 2,3,7,5,7,7,9,187
%N a(n) = smallest m >= n such that the decimal concatenation n||n+1||n+2||...||m is prime, or -1 if no such prime exists.
%C These are the values of n+k from A341715. See that entry for more information.
%C See A084559 (the same for nonprime indices) for further information. - _M. F. Hasler_, Feb 22 2021
%C Suppose n > 3. If n == 2 (mod 3), then a(n) == 3 or 5 (mod 6). If n = 0 or 1 (mod 3), then a(n) == 1 (mod 6) (see A341716). - _Chai Wah Wu_, Feb 22 2021
%F a(n) = n iff n is prime, otherwise a(n) = A084559(n). - _M. F. Hasler_, Feb 22 2021
%t Array[Block[{k = #, s = #}, While[! PrimeQ[s], k++; s = FromDigits[IntegerDigits[s]~Join~IntegerDigits[k]]]; k] &, 8, 2] (* _Michael De Vlieger_, Feb 22 2021 *)
%o (PARI) A341717(n)=if(isprime(n), n, A084559(n)) \\ _M. F. Hasler_, Feb 22 2021
%Y Cf. A084559, A140793, A341715, A341716.
%K nonn,base,more
%O 2,1
%A _N. J. A. Sloane_, Feb 22 2021