A259121 - OEIS

%I #12 Jul 08 2021 01:19:57

%S 1,6,48,102,192,366,1002,20364,37446,56082

%N Numbers k such that 7*R_k + 10^k - 6 is prime, where R_k = 11...11 is the repunit (A002275) of length k.

%C Also, numbers k such that (16*10^k - 61)/9 is prime.

%C Terms from Kamada data.

%C a(11) > 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/1/17771.htm#prime">Prime numbers of the form 177...771</a>.

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

%e For k=6, 7*R_6 + 10^k - 6 = 777777 + 10000000 - 6 = 1777771 which is prime.

%t Select[Range[100000], PrimeQ[(16*10^#-61)/9] &]

%Y Cf. A002275.

%K nonn,more,hard

%O 1,2

%A _Robert Price_, Jun 18 2015