A056694 - OEIS

%I #26 Sep 08 2022 08:45:01

%S 0,1,2,4,7,8,14,50,70,76,223,295,314,2089,2905,3394,3881,5113,6055,

%T 7253,7994,18172,18970,35005,69673

%N Numbers k such that 80*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (8*10^(k+1)-53)/9 is prime.

%C a(26) > 10^5. - _Robert Price_, Oct 31 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/88883.htm#prime">Prime numbers of the form 88...883</a>.

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

%F a(n) = A099422(n) - 1. [adapted by _Georg Fischer_, Jan 04 2021]

%t Do[ If[ PrimeQ[80*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]

%o (Magma) [n: n in [0..500] | IsPrime((8*10^(n+1)-53) div 9)]; // _Vincenzo Librandi_, Nov 01 2014

%Y Cf. A002275, A093166, A099422.

%K hard,nonn

%O 1,3

%A _Robert G. Wilson v_, Aug 10 2000

%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008

%E a(22)-a(25) from _Robert Price_, Oct 31 2014