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 A257027 Numbers k such that 7*R_(k+2) - 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 0, 2, 3, 9, 11, 18, 74, 131, 144, 161, 224, 282, 390, 398, 614, 791, 1313, 1866, 9708, 10544, 13292, 13394, 29703, 30779, 72446 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (646*10^n-7)/9 is prime. Terms from Kamada. a(26) > 10^5. Robert Price, Jul 31 2016 LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 717w. EXAMPLE For k=2, 7*R_4 - 6*10^2 = 7777 - 600 = 7177 which is prime. a(1) =0 associated with 71. a(2)=2 associated with 7177. a(3)=3 associated with 71777. a(4) = 9 associated with 71777777777 etc . - Robert Price, Jul 31 2016 MATHEMATICA Select[Range[0, 100000], PrimeQ[(646*10^#-7)/9 ] &] PROG (PARI) for(n=0, 200, if(isprime((646*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015 (MAGMA) [n: n in [0..300] | IsPrime((646*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015 CROSSREFS Cf. A002275, A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A074338 A111319 A109658 * A271548 A110350 A057569 Adjacent sequences:  A257024 A257025 A257026 * A257028 A257029 A257030 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 14 2015 EXTENSIONS a(25) from Robert Price, Jul 31 2016 STATUS approved

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Last modified February 24 17:26 EST 2020. Contains 332209 sequences. (Running on oeis4.)