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%I #20 Jul 08 2021 05:05:39
%S 1,2,4,19,28,73,203,220,274,292,470,763,1891,3307,7007,7306,9755,
%T 11395,39452,78242
%N Numbers k such that 9*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 (88*10^k - 7)/9 is prime.
%C a(21) > 10^5. - _Robert Price_, Nov 30 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/97777.htm#prime">Prime numbers of the form 977...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 9*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A093944 (corresponding primes).
%K hard,nonn
%O 1,2
%A _Robert G. Wilson v_, Aug 11 2000
%E a(18)-a(20) from _Robert Price_, Nov 30 2014
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