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%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
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