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 A293272 Numbers k such that (10^k - 19)/9 is prime. 0
 3, 4, 6, 9, 34, 36, 126, 141, 160, 286, 678, 1236, 3757, 4681, 4881, 6360, 6588, 7056, 7752, 41925, 43423, 43503, 47565, 52608, 55084, 76069, 199248 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For k > 1, numbers such that k-2 occurrences of the digit 1 followed by the digits 09 is prime (see Example section). a(28) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 1w09 EXAMPLE 4 is in this sequence because (10^4 - 19)/9 = 1109 is prime. Initial terms and primes associated: a(1) = 3, 109; a(2) = 4, 1109; a(3) = 6, 111109; a(4) = 9, 111111109; a(5) = 34, 1111111111111111111111111111111109; etc. MATHEMATICA Select[Range[2, 100000], PrimeQ[10^# - 19)/9] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A102934 A338061 A191699 * A192286 A242028 A254002 Adjacent sequences:  A293269 A293270 A293271 * A293273 A293274 A293275 KEYWORD nonn,more,hard AUTHOR Robert Price, Oct 06 2017 EXTENSIONS a(27) from Robert Price, Jan 02 2018 STATUS approved

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Last modified April 13 00:24 EDT 2021. Contains 342934 sequences. (Running on oeis4.)