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%I #26 Jul 08 2021 00:48:01
%S 0,2,3,9,15,18,36,63,114,225,405,482,1241,2018,9830,15795,22535,27752,
%T 41033,95574,131144,156221
%N Numbers k such that 2*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 (25*10^k - 7)/9 is prime.
%C a(21) > 10^5. - _Robert Price_, Dec 17 2014
%C a(23) > 2*10^5. - _Robert Price_, May 19 2018
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/27777.htm#prime">Prime numbers of the form 277...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 2*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 6000}]
%Y Cf. A002275, A093938.
%K hard,nonn
%O 1,2
%A _Robert G. Wilson v_, Aug 10 2000
%E a(15) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
%E a(16)-a(20) from _Robert Price_, Dec 17 2014
%E a(21)-a(22) from _Robert Price_, May 19 2018
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