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 A282505 Numbers k such that (5*10^k - 29)/3 is prime. 0
 1, 2, 3, 4, 5, 6, 12, 32, 63, 116, 154, 221, 267, 468, 605, 749, 1911, 7241, 7406, 7797, 9428, 11094, 43917, 44127, 58384, 131223, 181127 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 1 followed by k-2 occurrences of the digit 6 followed by the digits 57 is prime (see Example section). a(28) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 16w57. EXAMPLE 3 is in this sequence because (5*10^3 - 29)/3 = 1657 is prime. Initial terms and primes associated: a(1) = 1, 7; a(2) = 2, 157; a(3) = 3, 1657; a(4) = 4, 16657; a(5) = 5, 166657; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[(5*10^# - 29)/3] &] PROG (PARI) isok(k) = isprime((5*10^k - 29)/3); \\ Michel Marcus, Mar 06 2018 CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A218345 A059460 A125141 * A165304 A266642 A245450 Adjacent sequences:  A282502 A282503 A282504 * A282506 A282507 A282508 KEYWORD nonn,more,hard AUTHOR Robert Price, Feb 16 2017 EXTENSIONS a(26)-a(27) from Robert Price, Mar 05 2018 STATUS approved

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Last modified November 20 23:03 EST 2018. Contains 317427 sequences. (Running on oeis4.)