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%I #51 Apr 14 2024 03:00:39
%S 1,8,9,14,54,80,487,551,600,2502,2544,5593,7949,8635,13407,31128,
%T 45504,45933,52303,65121,167501,359354,642225,1029523,1170023
%N Numbers k such that 9*R_(k+2) - 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also, numbers k such that 93*10^k - 1 is prime.
%C Terms up to a(22) from Kamada.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/abaaa.htm">Near-repdigit numbers of the form ABAA...AA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/92999.htm#prime">Prime numbers of the form 9299...99</a>.
%H Predrag Kurtovic, <a href="https://t5k.org/primes/page.php?id=130796">93*10^642225 - 1</a>, The 5000 Largest Known Primes.
%H Predrag Kurtovic, <a href="https://t5k.org/primes/page.php?id=125948">93*10^1029523 - 1</a>, The 5000 Largest Known Primes.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e For k = 8, 9*R_10 - 7*10^8 = 9999999999 - 700000000 = 9299999999 which is prime, so 8 is a term.
%t Select[Range[0, 400000], PrimeQ[93*10^#-1 ] &]
%o (Magma) [n: n in [0..400] | IsPrime(93*10^n-1)]; // _Vincenzo Librandi_, Apr 15 2015
%Y Cf. A002275.
%K more,hard,nonn,changed
%O 1,2
%A _Robert Price_, Apr 14 2015
%E a(23) from _Predrag Kurtovic_, Dec 13 2020
%E a(24) from _Predrag Kurtovic_, Jan 29 2019
%E a(25) from Kamada data by _Tyler Busby_, Apr 14 2024
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