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 A102031 #25 Sep 08 2022 08:45:16 %S A102031 0,2,11,266,842,6299,37991,54116,121241,121620 %N A102031 Indices of primes in sequence defined by A(0) = 19, A(n) = 10*A(n-1) - 1 for n > 0. %C A102031 Numbers n such that (170*10^n + 1)/9 is prime. %C A102031 Numbers n such that digit 1 followed by n >= 0 occurrences of digit 8 followed by digit 9 is prime. %C A102031 Numbers corresponding to terms <= 842 are certified primes. %C A102031 a(11) > 2*10^5. - _Robert Price_, Nov 16 2014 %D A102031 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. %H A102031 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/18889.htm#prime">Prime numbers of the form 188...889</a>. %H A102031 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>. %F A102031 a(n) = A102945(n) - 1. %e A102031 1889 is prime, hence 2 is a term. %p A102031 A102031:=n->`if`(isprime((170*10^n+1)/9), n, NULL): seq(A102031(n), n=0..10^3); # _Wesley Ivan Hurt_, Nov 16 2014 %t A102031 Select[Range[0, 10^3], PrimeQ[(170*10^# + 1)/9] &] (* _Wesley Ivan Hurt_, Nov 16 2014 *) %o A102031 (PARI) a=19;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a-1) %o A102031 (PARI) for(n=0,1500,if(isprime((170*10^n+1)/9),print1(n,","))) %o A102031 (Magma) [n: n in [0..300] | IsPrime((170*10^n+1) div 9)]; // _Vincenzo Librandi_, Nov 17 2014 %Y A102031 Cf. A000533, A002275, A102945. %K A102031 nonn,hard,more %O A102031 1,2 %A A102031 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004 %E A102031 6299 from Herman Jamke (hermanjamke(AT)fastmail.fm), Dec 22 2007 %E A102031 a(7)-a(10) added by _Max Alekseyev_, Dec 12 2011