A259134 - OEIS

%I #12 Jul 08 2021 01:19:42

%S 0,2,4,28,156,322,352,1212,1284,7984,15192,84772,119930,148860

%N Numbers k such that 9*R_k + 7*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

%C Also, numbers k such that 8*10^k - 3 is prime.

%C Terms from Kamada data.

%C Note that Kamada does not recognize k=0 as 5 is a degenerate case of form ABB..BBA.

%C a(15) > 2*10^5.

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

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/79997.htm#prime">Prime numbers of the form 799...997</a>.

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

%e For k=2, 9*R_2 + 7*10^k - 2 = 99 + 700 - 2 = 797 which is prime.

%t Select[Range[0, 200000], PrimeQ[8*10^#-3] &]

%Y Cf. A002275.

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Jun 18 2015