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
Examples
127 is a member as both 1279 and 9127 are primes.
Links
- Robert Israel, Table of n, a(n) for n = 1..10000
Programs
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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);
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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