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 A287680 Numbers k such that 8*10^k + 81 is prime. 0
 0, 2, 3, 9, 11, 26, 39, 42, 58, 75, 105, 167, 279, 561, 2152, 2378, 3774, 5001, 5736, 9470, 13419, 14535, 32291, 89800, 146421 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 0 followed by the digits 81 is prime (see Example section). a(26) > 2*10^5. LINKS Makoto Kamada, Factorization of near-repdigit-related numbers. Makoto Kamada, Search for 80w81. EXAMPLE 3 is in this sequence because 8*10^3 + 81 = 8081 is prime. Initial terms and primes associated: a(1) = 0, 89; a(2) = 2, 881; a(3) = 3, 8081; a(4) = 9, 8000000081; a(5) = 11, 800000000081; etc. MATHEMATICA Select[Range[0, 100000], PrimeQ[(8*10^# + 81] &] CROSSREFS Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269. Sequence in context: A098016 A089645 A214259 * A242680 A275767 A088086 Adjacent sequences:  A287677 A287678 A287679 * A287681 A287682 A287683 KEYWORD nonn,more,hard AUTHOR Robert Price, May 29 2017 EXTENSIONS a(25) from Robert Price, Aug 22 2019 STATUS approved

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Last modified September 23 15:23 EDT 2021. Contains 347618 sequences. (Running on oeis4.)