A056664 - OEIS

%I #27 Jan 17 2019 13:44:05

%S 2,18,78,138,222,462,543,1095,1418,3246,3876,4416,9506,11090,14601,

%T 27810,29187

%N Numbers k such that 80*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (8*10^(k+1) - 71)/9 is prime.

%C There are no other terms <= 2500. - _Rick L. Shepherd_, Mar 02 2004

%C a(18) > 10^5. - _Robert Price_, Nov 01 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88881.htm#prime">Prime numbers of the form 88...881</a>.

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

%F a(n) = A099421(n+1) - 1. - _Robert Price_, Nov 01 2014

%t Do[ If[ PrimeQ[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}]

%Y Cf. A002275, A092675 (corresponding primes), A099421.

%K hard,nonn

%O 1,1

%A _Robert G. Wilson v_, Aug 09 2000

%E a(9) (giving a probable prime) from _Rick L. Shepherd_, Mar 02 2004

%E a(10)-a(15) from _N. J. A. Sloane_, Feb 20 2005

%E a(16)-a(17) derived from A099421 by _Robert Price_, Nov 01 2014