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 A272621 #7 Jun 13 2017 14:48:27 %S A272621 3,9,15,155,217,281,287,491,563,581,983,2243,4375,8409,98669 %N A272621 Numbers n such that 10^n-9^(n-1) is prime. %C A272621 a(16) > 10^5. %e A272621 3 is a member since 10^3-9^2 = 1000-81 = 919 which is a prime number. %t A272621 Select[Range[0, 100000], PrimeQ[10^# - 9^(# - 1)] &] %o A272621 (PARI) is(n)=ispseudoprime(10^n-9^(n-1)) \\ _Charles R Greathouse IV_, Jun 13 2017 %Y A272621 Cf. A093713, A082103, A093717, A093793, A096185, A093765, A093794, A093795, A096186. %K A272621 nonn,more %O A272621 1,1 %A A272621 _Robert Price_, May 03 2016