A256375 - OEIS

%I #30 Jul 08 2021 01:19:28

%S 4,5,6,8,13,14,25,30,44,58,383,455,463,504,511,1358,2293,4096,4374,

%T 8664,13676,23993,84134,90155,115794,127858

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

%C Also, numbers k such that (97*10^k - 1)/3 is prime.

%C Terms from Kamada.

%C a(27) > 250000.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>.

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/32333.htm#prime">Prime numbers of the form 3233...33</a>.

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

%e For k=4, 3*R_6 - 10^4 = 333333 - 10000 = 323333 which is prime.

%t Select[Range[0, 250000], PrimeQ[(97*10^#-1)/3 ] &]

%Y Cf. A002275.

%K more,hard,nonn

%O 1,1

%A _Robert Price_, Apr 14 2015