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 A283209 #14 Mar 06 2017 10:40:07
%S A283209 299777,299977,29999777,299999977,2999999777,299999999777,
%T A283209 2999977777777,299999999999977,2999999999977777777,
%U A283209 2999999999999999977,299999999999977777777,299999999999999999977,2999999999999999777777
%N A283209 Primes of the form 299...977...7 with at least one 9 and one 7.
%C A283209 If the number of 7's modulo 3 equals 1, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 3.
%C A283209 If the number of 9's modulo 6 equals 5, the corresponding 29..97..7 term cannot be in sequence because 299999 and 999999 are divisible by 7.
%C A283209 If the number of 7's and the number of 9's are both odd, the corresponding 29..97..7 term cannot be in sequence because it is divisible by 11.
%H A283209 Charles R Greathouse IV, <a href="/A283209/b283209.txt">Table of n, a(n) for n = 1..868</a>
%t A283209 Sort@ Select[Map[FromDigits@ Join[{2}, ConstantArray[9, #1], ConstantArray[7, #2]] & @@ # &, Select[Tuples[Range@ 20, 2], Times @@ Boole@ Map[OddQ, #] == 0 &]], PrimeQ] (* _Michael De Vlieger_, Mar 06 2017 *)
%o A283209 (PARI) do(n)=my(v=List(),p=29,q); for(d=3,n, p=10*p+7; q=p; forstep(i=d-3,1,-1, if(ispseudoprime(q+=2*10^i), listput(v,q)))); Vec(v) \\ _Charles R Greathouse IV_, Mar 06 2017
%K A283209 nonn,base
%O A283209 1,1
%A A283209 _FUNG Cheok Yin_, Mar 03 2017
%E A283209 More terms from _Charles R Greathouse IV_, Mar 06 2017