A056701 - OEIS

%I #37 May 31 2023 18:54:18

%S 0,1,2,3,4,10,16,22,53,91,94,106,138,210,282,522,597,1049,2227,6459,

%T 10582,18895,41269,50702,53185,59796,101395,116514,137551,153116

%N Numbers k such that 2*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (7*10^k - 1)/3 is prime.

%C a(31) > 3*10^5. - _Robert Price_, Oct 19 2014

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/23333.htm#prime">Prime numbers of the form 233...33</a>.

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

%t Do[ If[ PrimeQ[ 2*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 15001}]

%Y Cf. A002275, A093672.

%K hard,nonn,more

%O 1,3

%A _Robert G. Wilson v_, Aug 10 2000

%E a(22)-a(26) from Kamada data by _Robert Price_, Oct 19 2014

%E a(27)-a(30) from _Robert Price_, May 31 2023