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%I #17 Jul 08 2021 00:45:48
%S 0,1,11,17,773,18155
%N Numbers k such that 3*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 (34*10^k - 7)/9 is prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/3/37777.htm#prime">Prime numbers of the form 377...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 3*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 30070}]
%Y Cf. A002275, A093939.
%K hard,nonn
%O 1,3
%A _Robert G. Wilson v_, Aug 10 2000
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