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 A093947 #18 Jun 15 2025 17:30:37
%S A093947 7,79,79999,799999,799999999,79999999999,79999999999999999999999999,
%T A093947 79999999999999999999999999999999999999999999999999,
%U A093947 79999999999999999999999999999999999999999999999999999999999999999999999999999
%N A093947 Primes of the form 8*10^k - 1.
%C A093947 Equivalently, primes of the form 7*10^k + 9*R_k, where R_k is the repunit (A002275) of length k.
%H A093947 Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79999.htm#prime">Prime numbers of the form 799...99</a>.
%H A093947 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A093947 a(n) = 8*10^A056721(n) - 1 = A198973(A056721(n)). - _Elmo R. Oliveira_, Jun 14 2025
%t A093947 Select[Table[FromDigits[PadRight[{7}, n, 9]], {n, 50}], PrimeQ] (* _Vincenzo Librandi_, May 08 2019 *)
%o A093947 (Magma) [a: n in [0..200] | IsPrime(a) where a is (8*10^n-1)]; // _Vincenzo Librandi_, May 08 2019
%Y A093947 Cf. A002275, A056721 (corresponding k).
%Y A093947 Primes in A198973.
%K A093947 nonn
%O A093947 1,1
%A A093947 _Rick L. Shepherd_, Apr 17 2004