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%I #14 Oct 13 2024 09:49:38
%S 1,7,39,1203,10723,11971
%N Numbers k such that k*10^(k+1) + 1 is prime.
%C Numbers n such that the number that begins with n, followed by n zeros, ends with 1 is prime.
%C Those numbers corresponding to a(4), a(5) and a(6) are probable primes.
%C There is no further term up to 14000.
%C There is no further term up to 30000. - _Michael S. Branicky_, Mar 22 2023
%C There is no further term up to 50000. - _Michael S. Branicky_, Oct 13 2024
%t Do[If[PrimeQ[n*10^(n+1)+1],Print[n]],{n,14000}]
%o (PARI) is(n)=isprime(n*10^(n+1)+1) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A068817, A084428.
%K base,hard,more,nonn
%O 1,2
%A _Farideh Firoozbakht_, Sep 25 2009