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 A287208 Numbers k such that 8*10^k - 13 is prime. 0
 1, 2, 4, 7, 8, 13, 23, 28, 37, 40, 107, 132, 156, 290, 295, 653, 948, 2292, 4727, 6188, 7340, 24326, 34031, 41026, 66848, 104893, 106157, 125088 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 9 followed by the digits 87 is prime (see Example section). a(29) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 79w87. EXAMPLE 4 is in this sequence because 8*10^4 - 13 = 79987 is prime. Initial terms and primes associated: a(1) = 1, 67; a(2) = 2, 787; a(3) = 4, 79987; a(4) = 7, 79999987; a(5) = 8, 799999987; etc. MATHEMATICA Select[Range[1, 100000], PrimeQ[8*10^# - 13] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A190808 A018614 A176745 * A212243 A336463 A306235 Adjacent sequences:  A287205 A287206 A287207 * A287209 A287210 A287211 KEYWORD nonn,more,hard AUTHOR Robert Price, May 21 2017 EXTENSIONS a(26)-a(28) from Robert Price, Aug 08 2019 STATUS approved

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Last modified September 20 05:17 EDT 2021. Contains 347577 sequences. (Running on oeis4.)