%I #14 Aug 15 2019 07:28:52
%S 3,7,13,31,43,73,421,2551,6481,8191,250501,4002001,64008001,81009001,
%T 40000200001,90000300001,64000008000001,400000000020000000001,
%U 3600000000060000000001,64000000000008000000000001,90000000000000300000000000001,250000000000000500000000000001
%N Primes of the form b^2*10^(2*k) + b*10^k + 1 for 1 <= b <= 9, k >= 0.
%e b | Primes of the form b^2*10^(2*k) + b*10^k + 1
%e --+-------------------------------------------------------------
%e 1 | 3.
%e 2 | 7, 421, 4002001, 40000200001, 400000000020000000001, ...
%e 3 | 13, 90000300001, 90000000000000300000000000001, ...
%e 4 |
%e 5 | 31, 2551, 250501, 250000000000000500000000000001, ...
%e 6 | 43, 3600000000060000000001, ...
%e 7 |
%e 8 | 73, 6481, 64008001, 64000008000001, ...
%e 9 | 8191, 81009001, 810000000000000000900000000000000001, ...
%Y Numbers k such that b^2*10^(2*k) + b*10^k + 1 are prime: A297422 (b=2), A306751 (b=3), A308449 (b=5), A309582 (b=6), A309719 (b=8), A309744 (b=9).
%Y Cf. A309739.
%K nonn,base
%O 1,1
%A _Seiichi Manyama_, Aug 15 2019