A068678 Numbers that yield a prime when a 9 is prefixed or suffixed to them.

7, 19, 37, 41, 47, 71, 83, 91, 103, 127, 157, 161, 187, 199, 203, 209, 239, 257, 281, 293, 311, 349, 371, 377, 391, 413, 421, 433, 463, 467, 479, 491, 511, 539, 547, 551, 587, 619, 629, 661, 677, 689, 721, 749, 769, 787, 791, 803, 817, 859, 871, 883, 923

Offset: 1

Author

Amarnath Murthy, Mar 02 2002

Keywords

• base
• easy
• nonn

Examples

			127 is a member as both 1279 and 9127 are primes.
		

Links

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

Crossrefs

Cf. A068673, A068674, A068677.

Programs

Maple

i := 0:for k from 1 to 3200 do if(isprime(9+10*k) and isprime(k+10^(ceil(evalf(log(k+1)/log(10))))*9)) then s[i] := k; i := i+1; end if; end do:q := seq(s[j],j=0..i-1);

Mathematica

A068678Q[k_] := PrimeQ[10*k + 9] && PrimeQ[9*10^IntegerLength[k] + k];
Select[Range[1, 1000, 2], A068678Q] (* Paolo Xausa, Jul 03 2025 *)

Extensions

More terms from Sascha Kurz, Mar 17 2002