A068677 Numbers that yield primes when a 7 is prefixed or suffixed to them.

1, 3, 9, 19, 27, 33, 39, 57, 61, 87, 97, 109, 121, 127, 129, 159, 177, 187, 213, 229, 237, 243, 247, 283, 321, 351, 369, 417, 433, 451, 459, 481, 487, 507, 523, 529, 541, 547, 573, 589, 603, 621, 639, 673, 691, 699, 717, 723, 741, 753, 757, 793, 823, 829, 853

Offset

1,2

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

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

Maple

filter:= n -> isprime(10*n+7) and isprime(7*10^(1+ilog10(n))+n):
select(filter, [seq(i, i=1..1000, 2)]); # Robert Israel, Jul 01 2025

Mathematica

Flatten@Position[{# +7*10^IntegerLength@#, 7+10*#}&/@Range@10000, {_?PrimeQ ..}] (* Hans Rudolf Widmer, Aug 22 2024 *)
Select[Range[900], AllTrue[{7*10^IntegerLength[#]+#, 10 #+7}, PrimeQ]&] (* Harvey P. Dale, Feb 01 2026 *)

Crossrefs

Cf. A068673, A068674.

Sequence in context: A376220 A056682 A032670 * A069832 A057257 A029464

Adjacent sequences: A068674 A068675 A068676 * A068678 A068679 A068680

Keyword

base,easy,nonn

Author

Amarnath Murthy, Mar 02 2002

Extensions

More terms from Sascha Kurz, Mar 17 2002

Definition clarified by Harvey P. Dale, Feb 01 2026

Status

approved