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%I #25 Jul 08 2021 05:05:45
%S 2,9,15,32,38,65,123,173,257,320,326,639,719,774,902,15210,69999
%N Numbers k such that 8*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (79*10^k - 7)/9 is prime.
%C a(18) > 10^5. - _Robert Price_, Jan 11 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/87777.htm#prime">Prime numbers of the form 877...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 8*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A093943.
%K hard,nonn
%O 1,1
%A _Robert G. Wilson v_, Aug 11 2000
%E a(16)-a(17) from _Robert Price_, Jan 11 2015
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