This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A068678 #21 Jul 03 2025 07:24:19 %S A068678 7,19,37,41,47,71,83,91,103,127,157,161,187,199,203,209,239,257,281, %T A068678 293,311,349,371,377,391,413,421,433,463,467,479,491,511,539,547,551, %U A068678 587,619,629,661,677,689,721,749,769,787,791,803,817,859,871,883,923 %N A068678 Numbers that yield a prime when a 9 is prefixed or suffixed to them. %H A068678 Robert Israel, <a href="/A068678/b068678.txt">Table of n, a(n) for n = 1..10000</a> %e A068678 127 is a member as both 1279 and 9127 are primes. %p A068678 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); %t A068678 A068678Q[k_] := PrimeQ[10*k + 9] && PrimeQ[9*10^IntegerLength[k] + k]; %t A068678 Select[Range[1, 1000, 2], A068678Q] (* _Paolo Xausa_, Jul 03 2025 *) %Y A068678 Cf. A068673, A068674, A068677. %K A068678 base,easy,nonn %O A068678 1,1 %A A068678 _Amarnath Murthy_, Mar 02 2002 %E A068678 More terms from _Sascha Kurz_, Mar 17 2002