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 A257026 Numbers k such that 3*R_(k+2) + 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
 1, 2, 3, 9, 10, 13, 19, 25, 33, 124, 150, 175, 199, 290, 301, 506, 592, 1016, 1459, 4150, 6396, 7059, 8311, 8355, 8811, 13200, 13270, 26775, 33303, 62073, 66043, 154869, 162153, 172862 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also, numbers k such that (115*10^n-1)/3 is prime. Terms from Kamada. a(35) > 250000. LINKS Makoto Kamada, Near-repdigit numbers of the form ABAA...AA. Makoto Kamada, Prime numbers of the form 3833...33. EXAMPLE For k=2, 3*R_4 + 5*10^2 = 3333 + 500 = 3833 which is prime. MATHEMATICA Select[Range[0, 250000], PrimeQ[(115*10^#-1)/3 ] &] PROG (PARI) for(n=0, 300, if(isprime((115*10^n-1)/3), print1(n, ", "))) \\ Derek Orr, Apr 14 2015 (MAGMA) [n: n in [0..300] | IsPrime((115*10^n-1) div 3)]; // Vincenzo Librandi, Apr 15 2015 CROSSREFS Cf. A002275. Sequence in context: A039012 A291924 A214433 * A057291 A135107 A191568 Adjacent sequences:  A257023 A257024 A257025 * A257027 A257028 A257029 KEYWORD more,hard,nonn AUTHOR Robert Price, Apr 14 2015 STATUS approved

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Last modified April 6 10:28 EDT 2020. Contains 333273 sequences. (Running on oeis4.)