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 A056662 #19 Jul 04 2021 22:08:19
%S A056662 0,1,2,16,139,989,1886,3529,5995,13819,21872,26044,87719,232598
%N A056662 Numbers k such that 90*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A056662 Also numbers k such that 10^(k+1) - 3 is a prime number.
%H A056662 Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99997.htm#prime">Prime numbers of the form 99...997</a>.
%H A056662 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A056662 a(n) = A089675(n) - 1. - _Robert Price_, Nov 01 2014
%t A056662 Do[ If[ PrimeQ[ 90*(10^n - 1)/9 + 7 ], Print[n]], {n, 0, 1250}]
%Y A056662 Cf. A002275, A089675.
%K A056662 hard,nonn
%O A056662 1,3
%A A056662 _Robert G. Wilson v_, Aug 09 2000
%E A056662 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E A056662 a(13)-a(14) derived from A089675 by _Robert Price_, Nov 01 2014