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
Offset: 1
Examples
129 belongs to this sequence as both 1297 and 7129 are primes.
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Programs
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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
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Mathematica
Flatten@Position[{# +7*10^IntegerLength@#,7+10*#}&/@Range@10000,{?PrimeQ ..}] (* _Hans Rudolf Widmer, Aug 22 2024 *)
Extensions
More terms from Sascha Kurz, Mar 17 2002