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 A278470 #23 May 03 2024 15:42:24 %S A278470 13,20,40,43,73,85,576,1676,33565 %N A278470 Numbers k such that 10^k - 201 is prime. %C A278470 For k > 2, numbers k such that k-3 occurrences of the digit 9 followed by the digits 799 is prime. %C A278470 a(9) > 10000. - _Robert G. Wilson v_, Nov 24 2016 %H A278470 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>. %e A278470 13 is in this sequence because 10^13 - 201 = 9999999999799 is prime. %e A278470 Initial terms and associated primes: %e A278470 a(1) = 13, 9999999999799; %e A278470 a(2) = 20, 99999999999999999799; %e A278470 a(3) = 40, 9999999999999999999999999999999999999799; etc. %p A278470 A278470:=n->`if`(isprime(10^n-201), n, NULL): seq(A278470(n), n=1..10^3); # _Wesley Ivan Hurt_, Dec 08 2016 %t A278470 Select[Range[3, 2000], PrimeQ[10^# - 201] &] %o A278470 (Magma) [n: n in [3..500] | IsPrime(10^n-201)]; %o A278470 (PARI) is(n)=ispseudoprime(10^n-201) \\ _Charles R Greathouse IV_, Jun 06 2017 %Y A278470 Cf. A108327 (10^n-21), this sequence (10^n-201), A278397 (10^n-20001), A278471 (10^n-2001). %K A278470 nonn,more %O A278470 1,1 %A A278470 _Vincenzo Librandi_, Nov 23 2016 %E A278470 a(9) from Kamada data by _Tyler Busby_, May 03 2024