A095714 - OEIS

%I #35 Jul 08 2021 05:04:55

%S 3,5,7,33,45,105,197,199,281,301,317,1107,1657,3395,35925,37597,64305,

%T 80139,221631

%N Numbers k such that 9*R_k - 8 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that 10^k - 9 is a prime.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99991.htm#prime">Prime numbers of the form 99...991</a>.

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

%F a(n) = A056696(n) + 1.

%e a(2) = 5, since 10^5 - 9 = 99991, which is prime.

%t Do[ If[ PrimeQ[10^n - 9], Print[n]], {n, 0, 7000}]

%Y Cf. A002275, A093177, A056696.

%K nonn

%O 1,1

%A _Alonso del Arte_, Jul 07 2004

%E a(12) - a(14) from _Robert G. Wilson v_, Oct 15 2004

%E a(15) - a(16) from _Jason Earls_, Jan 07 2008

%E a(17) - a(19) from _Alexander Gramolin_, May 13 2011

%E Edited by _Ray Chandler_, Feb 26 2012

%E Title corrected by _Robert Price_, Sep 06 2014