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 A101700 #6 Mar 23 2022 15:19:18
%S A101700 21,291,2991,299999999999999991
%N A101700 Numbers of the form 3*(10^n-3), where 10^n-3 is prime.
%C A101700 a(5) = 3*(10^140-3) is 141 digits long and is too large to include.
%C A101700 If m is in this sequence then phi(m)=r(m), so this sequence is a subsequence of A069215. a(n)=3*(10^A089675(n)-3), so a(4)=3*(10^17-3), a(5)=3*(10^140-3), a(6)=3*(10^990-3), a(7)=3*(10^1887-3), a(8)=3*(10^3530-3), a(9)=3*(10^5996-3), a(10)=3*(10^13820-3), a(11)=3*(10^21873-3) & a(12)=3*(10 ^26045-3).
%F A101700 a(n) = 3*(10^A089675(n) - 3).
%e A101700 299999999999999991 is in the sequence because (10^17-3) is prime and 3*(10^17-3)=299999999999999991.
%t A101700 Do[If[PrimeQ[10^n-3], Print[3*(10^n-3)]], {n, 150}]
%t A101700 3#&/@Select[10^Range[20]-3,PrimeQ] (* _Harvey P. Dale_, Mar 23 2022 *)
%Y A101700 Cf. A086947, A089675, A069215.
%K A101700 nonn
%O A101700 1,1
%A A101700 _Farideh Firoozbakht_, Dec 31 2004