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 A101738 #15 Jan 17 2019 13:44:06 %S A101738 0,1,3,6,23,171,172,217,438,2562,2781,2879,3714,4391,10819,14591, %T A101738 25053,27768,40167,180991,193906 %N A101738 Indices of primes in sequence defined by A(0) = 41, A(n) = 10*A(n-1) + 81 for n > 0. %C A101738 Numbers n such that (450*10^n - 81)/9 is prime. %C A101738 Numbers n such that digit 4 followed by n >= 0 occurrences of digit 9 followed by digit 1 is prime. %C A101738 Numbers corresponding to terms <= 438 are certified primes. %C A101738 a(22) > 2*10^5. - _Robert Price_, May 30 2015 %D A101738 Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467. %H A101738 Makoto Kamada, <a href="https://stdkmd.net/nrr/4/49991.htm#prime">Prime numbers of the form 499...991</a>. %H A101738 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>. %F A101738 a(n) = A103001(n) - 1. %e A101738 49991 is prime, hence 3 is a term. %t A101738 Select[Range[0, 1000], PrimeQ[(450*10^# - 81)/9] &] (* _Robert Price_, May 30 2015 *) %o A101738 (PARI) a=41;for(n=0,1500,if(isprime(a),print1(n,","));a=10*a+81) %o A101738 (PARI) for(n=0,1500,if(isprime((450*10^n-81)/9),print1(n,","))) %Y A101738 Cf. A000533, A002275, A103001. %K A101738 nonn,hard,more %O A101738 1,3 %A A101738 _Klaus Brockhaus_ and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004 %E A101738 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 %E A101738 a(17)-a(19) from Kamada data by _Ray Chandler_, May 01 2015 %E A101738 a(20)-a(21) from _Robert Price_, May 30 2015