%I #10 Sep 08 2022 08:46:17
%S 1,7,15,21,91,325,465,853
%N Numbers n such that 7*10^n + 37 is prime.
%C All terms are odd because 7*(9*11+1)^n + 37 is divisible by 11.
%e 1 is in this sequence because 7*10 + 37 = 107 is prime.
%e 3 is not in the sequence because 7*10^3 + 37 = 31*227.
%e Initial terms and associated primes:
%e a(1) = 1: 107;
%e a(2) = 7: 70000037;
%e a(3) = 15: 7000000000000037;
%e a(4) = 21: 7000000000000000000037, etc.
%t Select[Range[0, 3000], PrimeQ[7 10^# + 37] &]
%o (Magma) [n: n in [1..400] | IsPrime(7*10^n+37)];
%o (PARI) lista(nn) = for(n=1, nn, if(ispseudoprime(7*10^n+37), print1(n, ", "))); \\ _Altug Alkan_, Jul 05 2016
%Y Cf. similar sequences listed in A274676.
%K nonn,more
%O 1,2
%A _Vincenzo Librandi_, Jul 04 2016
%E Edited by _Bruno Berselli_, Jul 05 2016