A068677 - OEIS

%I #15 Aug 23 2024 03:03:19

%S 1,3,9,19,27,33,39,57,61,87,97,109,121,127,129,159,177,187,213,229,

%T 237,243,247,283,321,351,369,417,433,451,459,481,487,507,523,529,541,

%U 547,573,589,603,621,639,673,691,699,717,723,741,753,757,793,823,829,853

%N Numbers that yield a prime when a 7 is prefixed or suffixed to them.

%e 129 belongs to this sequence as both 1297 and 7129 are primes.

%t Flatten@Position[{# +7*10^IntegerLength@#,7+10*#}&/@Range@10000,{_?PrimeQ ..}] (* _Hans Rudolf Widmer_, Aug 22 2024 *)

%Y Cf. A068673, A068674.

%K base,easy,nonn

%O 1,2

%A _Amarnath Murthy_, Mar 02 2002

%E More terms from _Sascha Kurz_, Mar 17 2002