A068677 - OEIS

A068677

Numbers that yield a prime 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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

Table of n, a(n) for n=1..55.

EXAMPLE

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

MATHEMATICA

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

CROSSREFS

Cf. A068673, A068674.

Sequence in context: A376220 A056682 A032670 * A069832 A057257 A029464