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 A068677 #19 Jul 02 2025 01:04:37
%S A068677 1,3,9,19,27,33,39,57,61,87,97,109,121,127,129,159,177,187,213,229,
%T A068677 237,243,247,283,321,351,369,417,433,451,459,481,487,507,523,529,541,
%U A068677 547,573,589,603,621,639,673,691,699,717,723,741,753,757,793,823,829,853
%N A068677 Numbers that yield a prime when a 7 is prefixed or suffixed to them.
%H A068677 Robert Israel, <a href="/A068677/b068677.txt">Table of n, a(n) for n = 1..10000</a>
%e A068677 129 belongs to this sequence as both 1297 and 7129 are primes.
%p A068677 filter:= n -> isprime(10*n+7) and isprime(7*10^(1+ilog10(n))+n):
%p A068677 select(filter, [seq(i,i=1..1000,2)]); # _Robert Israel_, Jul 01 2025
%t A068677 Flatten@Position[{# +7*10^IntegerLength@#,7+10*#}&/@Range@10000,{_?PrimeQ ..}] (* _Hans Rudolf Widmer_, Aug 22 2024 *)
%Y A068677 Cf. A068673, A068674.
%K A068677 base,easy,nonn
%O A068677 1,2
%A A068677 _Amarnath Murthy_, Mar 02 2002
%E A068677 More terms from _Sascha Kurz_, Mar 17 2002