A099421 - OEIS

%I #34 Apr 30 2024 05:27:17

%S 0,3,19,79,139,223,463,544,1096,1419,3247,3877,4417,9507,11091,14602,

%T 27811,29188,106729,188308

%N 0 together with numbers k such that 8*R_k - 7 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.

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

%C a(19) > 10^5. - _Robert Price_, Sep 06 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) = A056664(n-1) + 1.

%t Do[ If[ PrimeQ[ 8(10^n - 1)/9 - 7], Print[n]], {n, 0, 15000}]

%o (PARI)

%o for(n=0,10^4,if(ispseudoprime(abs(8*(10^n-1)/9-7)),print1(n,", "))) \\ _Derek Orr_, Sep 06 2014

%Y Cf. A002275, A056664.

%K nonn

%O 1,2

%A _Robert G. Wilson v_, Oct 14 2004

%E a(17)-a(18) from Kamada data by _Robert Price_, Sep 06 2014

%E a(19)-a(20) from Kamada data by _Tyler Busby_, Apr 30 2024