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%I #16 Mar 14 2023 16:25:19
%S 1,5,9,11,65,159
%N Numbers k such that 1.2. ... .k-1.k + 2 is prime (dot between numbers means concatenation).
%C Numbers k such that A007908(k)+2 is prime.
%C Next term is greater than 2000.
%C a(7) > 4034. - _J.W.L. (Jan) Eerland_, Nov 19 2022
%C a(7) > 15176. - _J.W.L. (Jan) Eerland_, Feb 24 2023
%e 9 is in the sequence because 123456789 + 2 is prime.
%t n=0;Monitor[Parallelize[While[True,If[PrimeQ[2+ToExpression[StringJoin[ToString/@Table[k,{k,1,n}]]]],Print[n]];n++];n],n] (* _J.W.L. (Jan) Eerland_, Nov 19 2022 *)
%Y Cf. A007908, A099081, A099083.
%K base,more,nonn
%O 1,2
%A _Farideh Firoozbakht_, Oct 23 2004
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