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 A068688 #14 Jan 28 2023 12:11:07
%S A068688 727,757,787,797,72227,75557,76667,78887,79997,7666667,722222227,
%T A068688 74444444447,75555555557,755555555555555555557,
%U A068688 75555555555555555555557,72222222222222222222222222227,79999999999999999999999999997,7444444444444444444444444444447
%N A068688 Primes which are a sandwich of numbers made of only one digit between two 7's.
%C A068688 The middle digit is never 0, 1, 3, or 7. - _Harvey P. Dale_, May 05 2018
%C A068688 a(40) has 1213 digits. - _Michael S. Branicky_, Jan 28 2023
%H A068688 Harvey P. Dale, <a href="/A068688/b068688.txt">Table of n, a(n) for n = 1..39</a>
%t A068688 Select[Flatten[Table[10FromDigits[PadRight[{7},n,i]]+7,{n,2,100},{i,9}]],PrimeQ] (* _Harvey P. Dale_, May 05 2018 *)
%o A068688 (Python)
%o A068688 from sympy import isprime
%o A068688 from itertools import count, islice
%o A068688 def agen(): yield from (t for i in count(1) for m in "0123456789" if isprime(t:=int("7" + m*i + "7")))
%o A068688 print(list(islice(agen(), 30))) # _Michael S. Branicky_, Jan 28 2023
%Y A068688 Cf. A068685, A068687, A068689.
%K A068688 base,nonn
%O A068688 1,1
%A A068688 _Amarnath Murthy_, Mar 02 2002
%E A068688 More terms from _Sascha Kurz_, Mar 17 2002