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 A089675 #50 Jun 07 2021 05:10:26 %S A089675 1,2,3,17,140,990,1887,3530,5996,13820,21873,26045,87720,232599, %T A089675 480684,538640 %N A089675 Numbers k such that 10^k - 3 is prime. %C A089675 Numbers k such that 9*R_k - 2 is a prime number, where R_k = 11...1 is the repunit (A002275) of length k. %C A089675 If k is in the sequence (10^k-3 is prime) and m=3*(10^k-3) then phi(m)=reversal(m), i.e., m is in the sequence A069215. - _Farideh Firoozbakht_, Dec 25 2004 %C A089675 No further terms for k <= 407197, see Kamada link. %H A089675 Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99997.htm#prime">Prime numbers of the form 99...997</a>. %H A089675 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>. %F A089675 a(n) = A056662(n) + 1. %e A089675 10^2 - 3 = 97 is a prime number (in fact all terms are the largest less than 10^k). %t A089675 m = 1000; For[n = 1, n < m, If[PrimeQ[10^n - 3], Print[n]]; n++] %Y A089675 Cf. A002275, A056662, A069215, A093172, A101700. %K A089675 more,nonn %O A089675 1,2 %A A089675 Michael Gottlieb (mzrg(AT)verizon.net), Jan 05 2004 %E A089675 a(8) from _Robert G. Wilson v_, Jan 14 2004 %E A089675 a(9) and a(10) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 06 2004 %E A089675 a(11) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 13 2004 %E A089675 a(12) from _Henri Lifchitz_. %E A089675 Edited by _Patrick De Geest_, Dec 28 2004 %E A089675 Edited by _Ray Chandler_, Dec 23 2010 %E A089675 a(15) from _Paul Bourdelais_, Jan 06 2021 %E A089675 a(16) from _Paul Bourdelais_, Jan 28 2021