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 A101823 #24 Aug 18 2025 02:15:04 %S A101823 0,2,6,9,27,35,66,80,146,482,642,1019,1899,2619,10452,27719,52823, %T A101823 105588,111987,618852,665828 %N A101823 Indices of primes in sequence defined by A(0) = 31, A(n) = 10*A(n-1) - 9 for n > 0. %C A101823 Numbers n such that 30*10^n + 1 is prime. %C A101823 Numbers n such that digit 3 followed by n >= 0 occurrences of digit 0 followed by digit 1 is prime. %D A101823 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. %H A101823 Makoto Kamada, <a href="https://stdkmd.net/nrr/3/30001.htm#prime">Prime numbers of the form 300...001</a>. %F A101823 a(n) = A056807(n) - 1. %e A101823 3001 is prime, hence 2 is a term. %o A101823 (PARI) a=31;for(n=0,2000,if(isprime(a),print1(n,","));a=10*a-9) %o A101823 (PARI) for(n=0,2000,if(isprime(30*10^n+1),print1(n,","))) %Y A101823 Cf. A000533, A002275, A056807. %K A101823 nonn,more,hard %O A101823 1,2 %A A101823 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 20 2004 %E A101823 More terms from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008 %E A101823 a(18) from Roman Makarchuk, Dec 05 2008 confirmed as next term by _Ray Chandler_, Mar 02 2012 %E A101823 a(19) from _Alexander Gramolin_, Feb 24 2012 confirmed as next term by _Ray Chandler_, Mar 02 2012 %E A101823 a(20)-a(21) derived from A056807 by _Robert Price_, Jan 26 2015