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%I #24 Mar 03 2024 04:14:37
%S 0,1,3,4,7,22,28,39,130,135,214,610,766,2152,2575,22972,42688,85711,
%T 85863,112066,538507,631714
%N Numbers k such that 10*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (10^(k+1)+53)/9 is prime.
%C 2575 also produces a probable prime.
%C a(20) > 10^5. - _Robert Price_, Jan 13 2015
%C a(23) > 670000 (per the Kamada link). - _Bill McEachen_, Mar 02 2024
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/11117.htm#prime">Prime numbers of the form 11...117</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A097684(n) - 1 for all n >= 0. - _Rick L. Shepherd_, Aug 23 2004
%t Do[ m = n; If[ primeQ[ 10*(10^n - 1)/9 + 7 ], Print[ n ] ], {n, 1, 1250} ]
%Y Cf. A093139 (corresponding primes), A097684.
%K hard,nonn
%O 1,3
%A _Robert G. Wilson v_, Aug 09 2000
%E 2152 (giving a probable prime) from _Rick L. Shepherd_, Mar 23 2004
%E 2575 from _Rick L. Shepherd_, Aug 23 2004
%E a(16)-a(19) derived from A097684 by _Robert Price_, Jan 13 2015
%E a(20)-a(22) from the Kamada link by _Bill McEachen_, Mar 02 2024
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