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 A093945 #11 Jan 17 2019 13:44:06
%S A093945 499,4999,49999,4999999,499999999999999,
%T A093945 4999999999999999999999999999999999999999999999999999999
%N A093945 Primes of the form 5*10^n - 1.
%C A093945 Equivalently, primes of the form 4*10^n + 9*R_n, where R_n is the repunit (A002275) of length n.
%C A093945 If m is in the sequence then m appears at the end of m^3, in fact if n>1 and m=5*10^n-1 then m appears at the end of m^3. - _Farideh Firoozbakht_, Nov 10 2005
%C A093945 If n is in the sequence then 4n is a term of A067206. Namely the digits of 4n end in phi(4n) - the proof is easy. - _Farideh Firoozbakht_, Dec 30 2006
%C A093945 The next term -- a(7) -- has 211 digits. - _Harvey P. Dale_, Feb 20 2016
%H A093945 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/49999.htm#prime">Prime numbers of the form 499...99</a>.
%H A093945 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%t A093945 Select[Table[FromDigits[PadRight[{4},n,9]],{n,60}],PrimeQ] (* _Harvey P. Dale_, Feb 20 2016 *)
%Y A093945 Cf. A056712 (corresponding n).
%Y A093945 Cf. A067206.
%K A093945 nonn
%O A093945 1,1
%A A093945 _Rick L. Shepherd_, Apr 17 2004