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%I #61 Oct 25 2023 16:04:32
%S 1,2,3,4,7,8,33,38,157,252,359,365,567,876,3108,5780,12987,14984,
%T 22287,31574,37473,40984,49806,51364,62451
%N Numbers k such that 3*R_k + 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that (10^k + 119)/3 is prime.
%C Terms from Kamada data. Note Kamada does not recognize k=1 as 43 is a degenerate case of form AAA..ABA.
%C a(26) > 3*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/33373.htm#prime">Prime numbers of the form 33...3373</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k=3, 3*R_3 + 40 = 333 + 40 = 373 which is prime.
%t Select[Range[0, 250000], PrimeQ[(10^# + 119)/3] &]
%o (PARI) is(n)=ispseudoprime(10^n\3+40) \\ _Charles R Greathouse IV_, Apr 10 2015
%o (Magma) [n: n in [0..400] | IsPrime((10^n+119) div 3)]; // _Vincenzo Librandi_, Apr 11 2015
%Y Cf. A002275.
%K more,hard,nonn
%O 1,2
%A _Robert Price_, Apr 10 2015
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