%I #16 Jan 17 2019 13:44:08

%S 2,3,17,19,31,37,43,91,339,367,407,1135,1175,8653,11987,15793

%N Numbers n such that 10^n - 81 is prime.

%C The next term, if one exists, is >100000. - _Robert Price_, Apr 25 2011

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99919.htm#prime">Prime numbers of the form 99...9919</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%e 10^3 - 81 = 919 which is a prime number.

%t nMax=1000; Do[If[PrimeQ[10^n-81], Print[n]], {n, nMax}]

%o (PARI) is(n)=ispseudoprime(10^n-81) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A108330, A108328.

%K more,nonn

%O 1,1

%A _Robert Price_, Dec 21 2010